[media] media: ov7670: Allow 32x maximum gain for yuv422

4x gain ceiling is not enough to capture a decent image in conditions
of total darkness and only a LED light source. Allow a maximum gain
of 32x instead.
This doesn't have any drawback since the image quality in 'normal'
light conditions is the same.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>