spi/xilinx: Code cleanup

spi/xilinx: Code cleanup

On the transmission loop, check for remaining bytes at the loop
condition.

This way we can handle transmissions of 0 bytes and clean the code.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>