[media] gspca/sn9c20x: Test if sensor is a OV sensor

Instead of just assuming a ov9650 sensor based on USB ID,
double-check it, by reading the sensor ID.

Acked-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>