TTY: hso, use tty from tty_port

We switched tty refcounting there to the one provided by tty_port
helpers. So tty_port->tty is now protected by tty_port->lock, not by
hso_serial->serial_lock.

Side note: tty->driver_data does not need the lock, so it is needed
neither in open, nor in close paths.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jan Dumon <j.dumon@option.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>