media: cec-notifier: rename variables, check kstrdup and n->conn_name

dev -> hdmi_dev
conn -> conn_name

Check if n->conn_name is not NULL before calling strcmp.
Check the result of kstrdup, and clean up on error.

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