[media] v4l2-controls.txt: update to the new way of accessing controls

The way current and new values are accessed has changed. Update the
document to bring it up to date with the code.

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