powerpc/vio: Clarify vio_find_node() reference counting

Add comment clarifying that vio_find_node() takes a reference to the
embedded struct device which needs to be dropped after use.
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/vio.c

@@ -1648,6 +1648,9 @@ static struct vio_dev *vio_find_name(const char *name)
 /**
  * vio_find_node - find an already-registered vio_dev
  * @vnode: device_node of the virtual device we're looking for
+ *
+ * Takes a reference to the embedded struct device which needs to be dropped
+ * after use.
  */
 struct vio_dev *vio_find_node(struct device_node *vnode)
 {