V4L/DVB: gspca_stv06xx: Add support for camera button

Only tested with an stv6422 based cam, as that is the only stv06xx cam
I have with a button.

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