USB: core: of: document reference taken by companion helper

Document that the new companion-device lookup helper takes a reference
to the companion device which needs to be dropped after use.
Signed-off-by: NJohan Hovold <johan@kernel.org>
Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/of.c

@@ -53,6 +53,9 @@ EXPORT_SYMBOL_GPL(usb_of_get_child_node);
  *
  * Find the companion device from platform bus.
  *
+ * Takes a reference to the returned struct device which needs to be dropped
+ * after use.
+ *
  * Return: On success, a pointer to the companion device, %NULL on failure.
  */
 struct device *usb_of_get_companion_dev(struct device *dev)