TTY: n_tty, propagate n_tty_data

In some funtions we need only n_tty_data, so pass it down directly in
case tty is not needed there.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>