media: cec-notifier: make cec_notifier_get_conn() static

This function is no longer used by other drivers, so it can be
made static.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>