[media] V4L: sh_mobile_ceu_camera: implement .stop_streaming()

The .stop_streaming() videobuf2 operation has to be implemented to
guarantee, that video buffers are not written to after a STREAMOFF.

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