Merge #645

645: stm32 usart: Fix RX interrupt flag handling r=lulf a=timokroeger

* On v1 interrupts cannot be cleared individually.
  Instead they are cleared implicitly by reading or writing DR (which we do now).
* Multiple error flags can be set at the same time:
  Handle them all in one go intstead of re-entering the ISR for each one so that
  we do not lose any error flags on v1 hardware.
* Wake when the RX buffer becomes full: This allows fast running chips to pull data
  from the buffer before receiving the next byte.

Tested on v1 hardware, lets see if v2 still succeeds on CI.

Co-authored-by: Timo Kröger <[email protected]>

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