net: hso: fix NULL-deref on disconnect regression

Commit 8a12f883 ("net: hso: fix null-ptr-deref during tty device
unregistration") fixed the racy minor allocation reported by syzbot, but
introduced an unconditional NULL-pointer dereference on every disconnect
instead.

Specifically, the serial device table must no longer be accessed after
the minor has been released by hso_serial_tty_unregister().

Fixes: 8a12f883

 ("net: hso: fix null-ptr-deref during tty device unregistration")
Cc: stable@vger.kernel.org
Cc: Anirudh Rayabharam <mail@anirudhrb.com>
Reported-by: Leonardo Antoniazzi <leoanto@aruba.it>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Anirudh Rayabharam <mail@anirudhrb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>