Hardwareserial: Disable interrupts while setting TXC bit

[NicoHood]

As discussed here the TXC bit setting should be atomic. Otherwise other (non usart) interrupts may corrupt the hardwareserial flush() function.

[NicoHood]

seems to be the same issue as #3745 with already opened pr by me long time ago #3865

[matthijskooijman]

I'm closing this as a duplicate, since though even though #3745 was opened for a different issue (double transmissions), its discussion has broadened to include this flush lockup issue too.