for TX transfer completion as for reasons unknown this occasionally
causes SPI_SR_RXBUFF and SPI_SR_ENDRX to not rise.
In any case, once the RX part of the transfer is done it's obvious
that the preceding TX part had finished and checking of SPI_SR_TXEMPTY
was introduced to rule out a possible cause for the data corruption
mentioned in r236495 but which didn't turn out to be the problem
anyway.
MFC after: 3 days
8b8a1c1ea5