[media] gspca_cpia1: Add support for button

Not only the qx3 microscope has a button, but some cameras too. Tested
with the Trust sp@cecam 100 (and with a creative and ezcam without button).

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