s5p-mfc: Use NULL instead of 0 for pointer

s5p-mfc: Use NULL instead of 0 for pointer

<UPSTREAM MERGE NOT REQUIRED>

linux-media commit 0e5d61d87b3a1ad5591e0dfbe4c548f862e9f5a6
Original commit message as follows:

  Fixes the following warning:
  drivers/media/video/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>
Change-Id: I94dadee385729b0bd35639d6a1f3b3f937370499
Signed-off-by: John Sheu <sheu@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/45098