[media] v4l2-ctrls: introduce call_op define

Add the call_op define to safely call the control ops. This also allows
for controls without any ops such as the 'control class' controls.

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