media: i2c: rdacm20: Constify static structs

The only usage of rdacm20_video_ops is to assign it to the video field
in the v4l2_subdev_ops struct which is a pointer to const,  and the only
usage of rdacm20_subdev_ops is to pass its address to
v4l2_i2c_subdev_init() which accepts a pointer to const. Make them const
to allow the compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>