[media] gscpa: Use v4l2_fh and add G/S_PRIORITY support

In order to support control event gspca has to use struct v4l2_fh.
As a bonus feature this also gives priority handling for free.

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