[media] V4L: Use NULL pointer instead of plain integer in v4l2-ctrls.c file

Fixes the following sparse warning:
drivers/media/video/v4l2-ctrls.c:2123:43: 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: Mauro Carvalho Chehab <mchehab@redhat.com>