[media] omap3isp: resizer: Center the crop rectangle

When the crop rectangle needs to be modified to match hardware
requirements, center the resulting rectangle on the requested rectangle
instead of removing pixels from the right and bottom sides only.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>