spi: spi-topcliff-pch: Fix sparse warnings

Fixes the following type of warnings:
drivers/spi/spi-topcliff-pch.c:370:52: warning:
Using plain integer as NULL pointer

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