[media] soc_camera: Constify dev_pm_ops in mt9t031.c

Silences the following warning:
WARNING: struct dev_pm_ops should normally be const

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>