V4L/DVB: gspca_mr97310a: Simplify sensor detection

Simplify sensor detection a bit (share more of the code between cif and vga
cameras). Also remove an unnecessary write from zero_the_pointer().

Signed-off-by: Theodore Kilgore <kilgota@banach.math.auburn.edu>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>