[media] v4l2-ioctl.c: fix overlay support

The vidioc_overlay op needs an unsigned int, not an unsigned int pointer.
So we need a small function that dereferences the unsigned int pointer.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>