[media] media: sh_mobile_ceu_camera: fix const cropping related warnings

A recent commit "[media] v4l2: make vidioc_s_crop const" introduced
warnings in sh_mobile_ceu_camera. Fix them by cleanly separating writable
and read-only variables in cropping operations.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>