Potential typo/bug in HardwareSerial::_tx_udr_empty_irq?

I noticed the following code in `HardwareSerial::_tx_udr_empty_irq`:

https://github.com/arduino/ArduinoCore-avr/blob/855ea0100ba0f6b9036a19a6e71517125473b014/cores/arduino/HardwareSerial.cpp#L103-L107

Shouldn't

```
*_ucsra = ((*_ucsra) & ((1 << U2X0) | (1 << TXC0)));
```

be

```
*_ucsra = ((*_ucsra) & ((1 << U2X0))) | (1 << TXC0);
```

just like the line in `#ifdef MPCM0`?

Because now it preserves the `TXC0` bit instead of setting it?

Start in cores/arduino/HardwareSerial.cpp at HardwareSerial::_tx_udr_empty_irq and compare the referenced assignments, including the #ifdef MPCM0 variant. Check the AVR register semantics and the open pull request #603 before deciding whether the behavior is incorrect; done means the issue is resolved consistently with the intended TXC0 and U2X0 handling.

Written by the indexing model from the issue text.