spi/xilinx: Use cached value of register

The control register has not changed since the previous access.
Therefore we can use the cached value and safe one bus access.

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