wifi: mt76: mt7921: fix kernel panic by accessing invalid 6GHz channel info

When the chip not support 6GHz capability, the channels of 6GHz information
should not be updated.

May get the stacktrace without this patch.
<1>[   19.442078] Unable to handle kernel NULL pointer dereference at
virtual address 0000000000000014
<1>[   19.457535] Mem abort info:
<1>[   19.465329]   ESR = 0x0000000096000004
<1>[   19.473295]   EC = 0x25: DABT (current EL), IL = 32 bits
<1>[   19.482354]   SET = 0, FnV = 0
<1>[   19.489143]   EA = 0, S1PTW = 0
<1>[   19.495991]   FSC = 0x04: level 0 translation fault
<1>[   19.504554] Data abort info:
<1>[   19.511111]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
<1>[   19.520269]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
<1>[   19.528988]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
<1>[   19.537960] user pgtable: 4k pages, 48-bit VAs, pgdp=00000001027a9000
<1>[   19.548014] [0000000000000014] pgd=0000000000000000, p4d=000000000000
<0>[   19.558429] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
<4>[   19.568270] Modules linked in: mt7921e mt7921_common mt792x_lib
mt76_connac_lib mt76 mac80211 btusb btintel cfg80211 btmtk snd_sof_ipc_msg_
btrtl snd_sof_ipc_flood_test btbcm bluetooth snd_sof_mt8195 uvcvideo
mtk_adsp_common snd_sof_xtensa_dsp uvc snd_sof_of snd_sof videobuf2_vmalloc
ecdh_generic ecc snd_sof_utils cros_ec_lid_angle cros_ec_sensors crct10dif_
cros_ec_sensors_core cros_usbpd_logger crypto_user fuse ip_tables ipv6
<4>[   19.614237] CPU: 1 PID: 105 Comm: kworker/1:1 Not tainted
6.6.0-rc6-next-20231017+ torvalds#324
<4>[   19.625957] Hardware name: Acer Tomato (rev2) board (DT)
<4>[   19.634970] Workqueue: events mt7921_init_work [mt7921_common]
<4>[   19.644522] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTY
<4>[   19.655182] pc : mt7921_regd_notifier+0x180/0x290 [mt7921_common]
<4>[   19.664983] lr : mt7921_regd_notifier+0xd4/0x290 [mt7921_common]
<4>[   19.674679] sp : ffff800080acba80
<4>[   19.681649] x29: ffff800080acba80 x28: 0000000000000000 x27: ffff4faf
<4>[   19.692483] x26: 0000000000000000 x25: 0000000000000000 x24: ffff4faf
<4>[   19.703294] x23: 00000000ffffe926 x22: ffff4faf16031fa0 x21: 00000000
<4>[   19.714108] x20: 000000000000001c x19: ffff4faf16ba6f40 x18: 00000000
<4>[   19.724928] x17: 0000000000000000 x16: ffffac6b891c2750 x15: ffff8000
<4>[   19.735722] x14: 0000000000000180 x13: 0000000000000000 x12: 00000000
<4>[   19.746478] x11: 0000000000000002 x10: ffff4faf01c21780 x9 : ffffac6b
<4>[   19.757214] x8 : 00000000006c0000 x7 : ffffac6b6b020cf0 x6 : ffffac6b
<4>[   19.767945] x5 : ffffac6b6b020d00 x4 : ffffac6b6b020cf8 x3 : ffff4faf
<4>[   19.778648] x2 : 0000000000000000 x1 : 000000000000001c x0 : 00000000
<4>[   19.789366] Call trace:
<4>[   19.795381]  mt7921_regd_notifier+0x180/0x290 [mt7921_common]
<4>[   19.804675]  wiphy_update_regulatory+0x2bc/0xa08 [cfg80211]
<4>[   19.813864]  wiphy_regulatory_register+0x4c/0x88 [cfg80211]
<4>[   19.823029]  wiphy_register+0x75c/0x8d0 [cfg80211]
<4>[   19.831446]  ieee80211_register_hw+0x70c/0xc10 [mac80211]
<4>[   19.840479]  mt76_register_device+0x168/0x2e8 [mt76]
<4>[   19.849008]  mt7921_init_work+0xdc/0x250 [mt7921_common]
<4>[   19.857817]  process_one_work+0x148/0x3c0
<4>[   19.865292]  worker_thread+0x32c/0x450
<4>[   19.872489]  kthread+0x11c/0x128
<4>[   19.879173]  ret_from_fork+0x10/0x20
<0>[   19.886153] Code: f0000041 9100a021 94000aef aa0003f9 (b9401780)
<4>[   19.895634] ---[ end trace 0000000000000000 ]---

Reported-by: AngeloGioacchino Del Regno <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Fixes: 09382d8 ("mt7921: update the channel usage when the regd domain changed")
Signed-off-by: Ming Yen Hsieh <[email protected]>
Signed-off-by: Deren Wu <[email protected]>

Author: Ming Yen Hsieh

drivers/net/wireless/mediatek/mt76/mt7921/init.c

@@ -88,6 +88,9 @@ mt7921_regd_channel_update(struct wiphy *wiphy, struct mt792x_dev *dev)
 	}
 
 	sband = wiphy->bands[NL80211_BAND_6GHZ];
+	if (!sband)
+		return;
+
 	band_np = np ? of_get_child_by_name(np, "txpower-6g") : NULL;
 	for (i = 0; i < sband->n_channels; i++) {
 		ch = &sband->channels[i];