[media] ov7670: check for valid width/height in ov7670_enum_frame_interval

The width and height should be checked in the enum_frame_interval
op. This fixes a v4l2-compliance failure.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>