TTY: isdn, use tty from tty_port

No recounting this time, just a plain switch.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>