V4L/DVB (13139): gspca_mr97310a: Improve VGA sensor type detection

Improve (and also simplify :) gspca_mr97310a: VGA sensor type detection.
As it was still failing on some machines (not with some cams, but on
some machines interesting enough).

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>