[media] s5p-mfc: Use NULL instead of 0 for pointer

Fixes the following warning:
drivers/media/platform/s5p-mfc/s5p_mfc_opr.c:56:27: warning:
Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>