[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>

drivers/media/video/v4l2-ctrls.c

@@ -2121,7 +2121,7 @@ static int prepare_ext_ctrls(struct v4l2_ctrl_handler *hdl,
 
 	/* First zero the helper field in the master control references */
 	for (i = 0; i < cs->count; i++)
-		helpers[i].mref->helper = 0;
+		helpers[i].mref->helper = NULL;
 	for (i = 0, h = helpers; i < cs->count; i++, h++) {
 		struct v4l2_ctrl_ref *mref = h->mref;