spi/xilinx: Remove rx_fn and tx_fn pointer

Simplify the code by removing the tx and and rx function pointers and
substitute them by a single function.

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

diff --cc drivers/spi/spi-xilinx.c
Simple merge