[media] gspca: clear priv field and disable relevant ioctls

The v4l2_pix_format priv field must be 0, so zero it.

Also disable ioctls that are not implemented by a subdriver.

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>