media: meson/ao-cec: use cec_notifier_cec_adap_(un)register

Use the new cec_notifier_cec_adap_(un)register() functions to
(un)register the notifier for the CEC adapter.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>