V4L/DVB: gspca_main: Allow use of input device creation code for non int. inputs

Allow use of the gspca core input device creation code by subdrivers which
have non interrupt driven camera buttons.

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