TTY: ipwireless, use tty from tty_port

It does not make the driver less racy though. Close and hangup should
be rewritten and tty refcounting used properly.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jiri Kosina <jkosina@suse.cz>
Acked-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>