[media] ov7670: fix QVGA visible area

The QVGA mode has a green horizontal line on the left hand side, and a red
(or sometimes blue) vertical line at the bottom. Tweak the visible area
to remove them.

Thanks to Mauro for explaining how to fix this.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>