[media] gspca_cpia1: Don't allow the framerate divisor to go above 2

Having a framerate divisor of 3 is not really necesarry and leads to
various troubles (frames going missing, compression issues) when allowed.

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