media: ov7740: constify structures stored in fields of v4l2_subdev_ops structure

The fields of a v4l2_subdev_ops structure are all const, so the
structures that are stored there and are not used elsewhere can be
const as well.

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+samsung@kernel.org>