[media] v4l2-ctrls: fix comments

Various comments referred to videodev2.h, but the control definitions have
been moved to v4l2-controls.h.

Also add the same reminder message to each class of controls.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>