[media] v4l2-ctrls: add helper functions for control events

Many drivers just support control events, and most radio drivers just need
to poll for control events. Add some functions to simplify those jobs.

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