[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/drivers/media/pci/ivtv/ivtv-ioctl.c b/drivers/media/pci/ivtv/ivtv-ioctl.c
index d3b32c2..966abb4 100644
--- a/drivers/media/pci/ivtv/ivtv-ioctl.c
+++ b/drivers/media/pci/ivtv/ivtv-ioctl.c
@@ -1460,7 +1460,7 @@
 	return 0;
 }
 
-static int ivtv_subscribe_event(struct v4l2_fh *fh, struct v4l2_event_subscription *sub)
+static int ivtv_subscribe_event(struct v4l2_fh *fh, const struct v4l2_event_subscription *sub)
 {
 	switch (sub->type) {
 	case V4L2_EVENT_VSYNC: