media: exynos4-is: constify video_subdev structures

The v4l2_subdev_ops structures are only passed as the second
argument of v4l2_subdev_init, which is const, so the
v4l2_subdev_ops structures can be const as well.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>