mmc: sdhci-s3c: Use NULL instead of 0 for pointers

The third argument for of_get_property() is a pointer, hence pass
NULL instead of 0.

Fixes the following sparse warning:
sdhci-s3c.c:452:48: warning: Using plain integer as NULL pointer
sdhci-s3c.c:457:52: warning: Using plain integer as NULL pointer

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>