tda9950: 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: Dariusz Marcinkiewicz <darekm@google.com>
Tested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20190814104520.6001-5-darekm@google.com