usb: usb251xb: add pm_ops

usb: usb251xb: add pm_ops

Currently the driver don't support pm_ops. These ops are not necessary
if the supply isn't switchable (always on). This assumptions seems to be
wrong because no one needs a powered hub during suspend-to-ram/disk.

So adding simple_dev_pm_ops to be able to switch off the hub during
suspend and to restore the config after a resume operation.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Acked-by: Richard Leitner <richard.leitner@skidata.com>
Link: https://lore.kernel.org/r/20190917144449.32739-5-m.felsch@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>