V4L/DVB: gspca_ov519: add support for the button on ov519 based cams

Note due to hardware limitiations (no interrupt endpoint), this only works
when the camera is streaming.

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