spi/xilinx: Check number of slaves range

The core only supports up to 32 slaves, and the chipselect function
expects the same.

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