V4L/DVB: gscpa_sonixb: limit ov7630 max framerate at 640x480

In 640x480, if the reg11 has less than 4, the image is
unstable (the bridge goes into a higher compression mode
which we have not reverse engineered yet).

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