[AX.25]: Fix unchecked ax25_listen_register uses

Fix ax25_listen_register to return something that's a sane error code,
then all callers to use it.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
3 files changed