[media] v4l2-ioctl: WARN_ON if querycap didn't fill device_caps

This is easy to forget to do in drivers. While v4l2-compliance will check for it,
not everyone remembers to run it. So warn about it.

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