media: mt9m111: constify video_subdev structures

The v4l2_subdev_ops structure is only passed as the third argument
of v4l2_i2c_subdev_init, which is const, so the v4l2_subdev_ops
structure can be const as well.  The other structures are only
stored in the v4l2_subdev_ops structure, all the fields of which are
const, so these structures can also be const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>