Clear serial error flags after reading them

[TheZoq2]

The current implementaion does not clear the error bits in the ISR register when running Rx.read(). This means that after an error has occured, all future reads will return the same error. The bits can be reset by writing to the ICR register which is what this patch does.

I'm a bit sceptical of the unsafe blocks, but they seem required as the Rx struct only contains a pointer to the USARTx registers.

I can also see this being implemented as a separate clear function which resets all error flags independently of read but I believe that would require modifying the embedded-hal crate, right?

[TheZoq2]

I made another update to this which always reads the content of the data register, even if an error occured. Without this, the next read would return the data that was sent after the error rather than no data at all.

However, im unsure wether it is safe to read data even if there are error flags. I have only tested it with overrrun errors.

[eeeeeta]

I just ran into this issue today when trying to use the USART; any update on how close this PR is to getting merged?

[TheZoq2]

Unfortunately for us, I believe @japaric is a bit inactive in this crate since he's working on more general embedded-wg things