spi: spi-xilinx: Remove redundant platform_set_drvdata()

Setting platform data to NULL is not necessary.
Also fixes the following sparse warning:
drivers/spi/spi-xilinx.c:508:35: warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>