fix uart_flush() returning early

[bertmelis]

wait for UART FSM to become idle before uart_flush() returns:
current situation:

new situation:

tested with a simple sketch:

#include <Arduino.h>

void setup() {
  Serial.begin(112500);
  pinMode(26, OUTPUT);
}

void loop() {
  static uint32_t lastMillis = 0;
  if (millis() - lastMillis > 10000) {
    lastMillis = millis();
    digitalWrite(26, HIGH);
    Serial.write("Hello World!");
    Serial.flush();
    digitalWrite(26, LOW);
  }
}

Tested with Serial, Serial1 and Serial2.

[stickbreaker]

@bertmelis That looks better!