[media] cec: add CEC_LOG_ADDRS_FL_ALLOW_UNREG_FALLBACK flag

Currently if none of the requested logical addresses can be claimed, the
framework will fall back to the Unregistered logical address.

Add a flag to enable this explicitly. By default it will just go back to
the unconfigured state.

Usually Unregistered is not something you want since the functionality is
very limited. Unless the application has support for this, it will fail
to work correctly. So require that the application explicitly requests
this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>