spi/xilinx: Make spi_tx and spi_rx simmetric

spi_rx handles the case where the buffer is null. Nevertheless spi_tx
did not handle it, and was handled by the caller function.

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