[media] gscpa: Move ctrl_handler to gspca_dev

We intend to eventually port all sub-drivers to the control-framework. At
which point it will make more sense to have the ctrl_handler in gspca_dev
then to have it in the subdrivers. Lets move it there now, to avoid a lot
of work to move it later.

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