USB: serial: do not use tty class device for debugging

Use the port struct device rather than tty class device for debugging.

Note that while USB serial doesn't support serdev yet (due to serdev not
handling hotplugging), serdev ttys do not have a corresponding class
device and would have been logged using a "(NULL device *):" prefix.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>