[media] media: mx3_camera: fix const cropping related warnings

A recent commit "[media] v4l2: make vidioc_s_crop const" introduced
warnings in mx3_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>