[media] v4l2: make vidioc_(un)subscribe_event const

Write-only ioctls should have a const argument in the ioctl op.
Do this conversion for vidioc_(un)subscribe_event.
Adding const for write-only ioctls was decided during the 2012 Media Workshop.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/include/media/v4l2-ctrls.h b/include/media/v4l2-ctrls.h
index 7ef6b27..6890f5e 100644
--- a/include/media/v4l2-ctrls.h
+++ b/include/media/v4l2-ctrls.h
@@ -546,7 +546,7 @@
 /* Can be used as a vidioc_subscribe_event function that just subscribes
    control events. */
 int v4l2_ctrl_subscribe_event(struct v4l2_fh *fh,
-				struct v4l2_event_subscription *sub);
+				const struct v4l2_event_subscription *sub);
 
 /* Can be used as a poll function that just polls for control events. */
 unsigned int v4l2_ctrl_poll(struct file *file, struct poll_table_struct *wait);