TTY: isdn, do not play with module refcounts

The module which called allocate_tty_driver is already refcounted by
the TTY layer automatically. And since THIS_MODULE is isdn_tty and it
allocated the tty_driver, there is no need to do the counts in isdn's
tty->ops->open/close.

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