USB: of: document reference taken by child-lookup helper

Document that the child-node lookup helper takes a reference to the
device-tree node which needs to be dropped after use.
Signed-off-by: NJohan Hovold <johan@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/of.c

@@ -28,7 +28,8 @@
  *
  * Find the node from device tree according to its port number.
  *
- * Return: On success, a pointer to the device node, %NULL on failure.
+ * Return: A pointer to the node with incremented refcount if found, or
+ * %NULL otherwise.
  */
 struct device_node *usb_of_get_child_node(struct device_node *parent,
 					int portnum)