TTY: n_tty, add ldisc data to n_tty

All n_tty related members from tty_struct will be moved here.

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>