media: v4l2_ctrl: Add const pointer to ctrl_ptr

This pointer is used to point to data that is constant. Thanks to this
we can avoid a lot of casting and we make more clear when the data is
constant or variable.

Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>