[media] omap3isp: video: Fix warning caused by bad vidioc_s_crop prototype

Commit 4f996594 ("v4l2: make vidioc_s_crop const") modified the
vidioc_s_crop operation prototype but forgot to update the OMAP3 ISP
driver. Add a const keyword to fix the function prototype.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>