usb: hub: Add a new API to test if a hub device is root hub

Sometimes we need know if a given hub device is root hub or not.
Add a new API to test this. This removes the xHCI driver's own
version is_root_hub() and change to use the new API.

While we are here, remove the unused/commented out get_usb_device()
in the xHCI driver too.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/usb.h b/include/usb.h
index a266677..64dfa84 100644
--- a/include/usb.h
+++ b/include/usb.h
@@ -776,6 +776,14 @@
 		     struct usb_device *parent);
 
 /**
+ * usb_hub_is_root_hub() - Test whether a hub device is root hub or not
+ *
+ * @hub:	USB hub device to test
+ * @return:	true if the hub device is root hub, false otherwise.
+ */
+bool usb_hub_is_root_hub(struct udevice *hub);
+
+/**
  * usb_hub_scan() - Scan a hub and find its devices
  *
  * @hub:	Hub device to scan