TTY: pdc_cons, use tty_port

Instead of digging a tty out of the tty_driver struct, which is not
defined to work, use tty_port properly. This includes proper tty
refcounting even though there is no possible race currently. But we
will need tty_port for tty buffers in the future anyway.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Cc: Helge Deller <deller@gmx.de>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>