MFC: r237239

MFC: r237239

Revert the part of r236495 (MFC'ed to stable/9 in r237095) that
introduced checking of SPI_SR_TXEMPTY 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.

git-svn-id: svn://svn.freebsd.org/base/stable/9@237382 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f