xhci: child detach fix

xhci_child_detach() zaps the wrong slot when unplugging a device
connected via usb-hub:  Instead of the device's slot the slot of the
usb-hub is used.  Fix it.

https://bugzilla.redhat.com/show_bug.cgi?id=1075846

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
