TTY: hvsi, use tty from tty_port

Now, we switch to the refcounted model and do not need hp->lock to
protect hp->tty anymore.

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