USB: mention URB_FREE_BUFFER in usb_free_urb documentation

The usb_free_urb comment says that the transfer buffer will not be
freed, but this is not the case when URB_FREE_BUFFER is set.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/urb.c

@@ -85,8 +85,8 @@ EXPORT_SYMBOL_GPL(usb_alloc_urb);
  * Must be called when a user of a urb is finished with it.  When the last user
  * of the urb calls this function, the memory of the urb is freed.
  *
- * Note: The transfer buffer associated with the urb is not freed, that must be
- * done elsewhere.
+ * Note: The transfer buffer associated with the urb is not freed unless the
+ * URB_FREE_BUFFER transfer flag is set.
  */
 void usb_free_urb(struct urb *urb)
 {