USB: correct the usb_disconnect() comment about usb_bus_list_lock

usb_disconnect() no longer acquires usb_bus_list_lock, so update its
comment to that effect.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hub.c

@@ -2023,8 +2023,8 @@ static void hub_free_dev(struct usb_device *udev)
  * Something got disconnected. Get rid of it and all of its children.
  *
  * If *pdev is a normal device then the parent hub must already be locked.
- * If *pdev is a root hub then this routine will acquire the
- * usb_bus_list_lock on behalf of the caller.
+ * If *pdev is a root hub then the caller must hold the usb_bus_list_lock,
+ * which protects the set of root hubs as well as the list of buses.
  *
  * Only hub drivers (including virtual root hub drivers for host
  * controllers) should ever call this.