Revert "usb: move struct usb_device->children to struct usb_hub_port->child"

This reverts commit bebc56d5.

The call here is fragile and not well thought out, so revert it, it's
not fully baked yet and I don't want this to go into 3.5.

Cc: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>