media: cec-adap: return from cec_s_conn_info() if adap is invalid

Check if cec_s_conn_info is called with a valid cec adapter,
do nothing if it is invalid.

This makes it possible to call this function even if CEC support is
disabled in the kernel config.

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