isdn: Remove ASYNC_CLOSING

The tty core no longer provides ASYNC_CLOSING. Use private flag for
same purpose, which is to disable AT-emulator output (why this is
necessary is not clear).

Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>