media: cec-adap.c: add 'unregistered' checks

Make the code a bit more robust by checking if the adapter has
been unregistered at the start of cec_transmit_msg_fh() and
cec_received_msg_ts(). If it is unregistered, then just return.

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