TTY: HVC, use tty from tty_port

The driver already used refcounting. So we just switch it to tty_port
helpers. And switch to tty_port->lock for tty.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>