TTY: isdn, use tty_port_close_end helper

The code does almost the same, so there we can leverage the helper's
code. The only difference is locking. The helper protects counts by a
spinlock. This never hurts and should be added to other code parts
too.

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