CHROMIUM: usb: xhci-plat: use usb_hcd_platform_shutdown() callback

CHROMIUM: usb: xhci-plat: use usb_hcd_platform_shutdown() callback

The xhci driver provides xhci_shutdown() to be called with the main
usb_hcd (the USB3 roothub) while do reboot. But actually, xhci-plat
never call xhci_shutdwon() during reboot because it doesn't use the
usb_hcd_platform_shutdown() helper.

So we use use usb_hcd_platform_shutdown() for xhci-plat, and the
normal shutdown call trace is:
kernel_restart -> kernel_restart_prepare -> device_shutdown ->
platform_drv_shutdown -> usb_hcd_platform_shutdown -> xhci_shutdown

BUG=chrome-os-partner:59111
TEST=reboot the system

Change-Id: I9be424257ea6ba1e51521cbdd01f4698ae1752ad
Signed-off-by: William wu <wulf@rock-chips.com>
Reviewed-on: https://chromium-review.googlesource.com/401119
Commit-Ready: Guenter Roeck <groeck@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>