[media] i2c: constify v4l2_ctrl_ops structures

These v4l2_ctrl_ops structures are never modified, like all the other
v4l2_ctrl_ops structures, so declare them as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>