V4L/DVB: V4L: Events: Replace bad WARN_ON() with assert_spin_locked()

spin_is_locked() always returns zero when spinlock debugging is
disabled on a single CPU machine. Replace WARN_ON() with
assert_spin_locked().

Thanks to Laurent Pinchart for spotting this!
Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/v4l2-event.c

@@ -152,7 +152,7 @@ static struct v4l2_subscribed_event *v4l2_event_subscribed(
 	struct v4l2_events *events = fh->events;
 	struct v4l2_subscribed_event *sev;
 
-	WARN_ON(!spin_is_locked(&fh->vdev->fh_lock));
+	assert_spin_locked(&fh->vdev->fh_lock);
 
 	list_for_each_entry(sev, &events->subscribed, list) {
 		if (sev->type == type)