NET: wan/x25_asy, move lapb_unregister to x25_asy_close_tty

We register lapb when tty is created, but unregister it only when the
device is UP. So move the lapb_unregister to x25_asy_close_tty after
the device is down.

The old behaviour causes ldisc switching to fail each second attempt,
because we noted for us that the device is unused, so we use it the
second time, but labp layer still have it registered, so it fails
obviously.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Sergey Lapin <slapin@ossfans.org>
Cc: Andrew Hendry <andrew.hendry@gmail.com>
Tested-by: Sergey Lapin <slapin@ossfans.org>
Tested-by: Mikhail Ulyanov <ulyanov.mikhail@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>