usb: hub: Use 'struct usb_hub_device' as hub device's uclass_priv

Use USB hub device's dev->uclass_priv to point to 'usb_hub_device'
so that with driver model usb_hub_reset() and usb_hub_allocate()
are no longer needed.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>