Serial Flush blocks forever but not only at programm start

Hi there

I have a similar issue to #597, but not at the start of my program.

In my code, there is a function which looks like this:

```
void SlaveWaterCOM::send() {

digitalWrite(2, HIGH);

Serial1.write(outbuffer.dataarray, 28);

Serial1.flush();

digitalWrite(2, LOW);
}
```

The function communicates via the Serial1 over a MAX3075, and therefore it needs an enablepin for the driver.

In my program, this is the only point I use the Serial1.write or the Serial1.flush command.

Sadly, the program sometimes hangs at the Serial. Flush() command.

I tracked the issue down to the Sercom->USART.INTFLAG.bit.TXC flag, which seems to never be written.

Has anyone a clue why this could be?

Start at the Serial1.flush() implementation and trace how it waits for Sercom->USART.INTFLAG.bit.TXC. Reproduce the intermittent hang with the MAX3075 driver-enable sequence shown in the issue, then determine why TXC is not set and document a verified fix or minimal reproduction.

Written by the indexing model from the issue text.