[media] media: ov7670: calculate framerate properly for ov7675

According to the datasheet ov7675 uses a formula to achieve
the desired framerate that is different from the operations
done in the current code.
In fact, this formula should apply to ov7670 too. This would
mean that current code is wrong but, in order to preserve
compatibility, the new formula will be used for ov7675 only.

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