of: Warn if of_graph_get_next_endpoint is called with the root node

If of_graph_get_next_endpoint is given a parentless node instead of an
endpoint node, it is clearly a bug.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>

drivers/of/base.c

@@ -2028,8 +2028,8 @@ struct device_node *of_graph_get_next_endpoint(const struct device_node *parent,
 		of_node_put(node);
 	} else {
 		port = of_get_parent(prev);
-		if (!port)
-			/* Hm, has someone given us the root node ?... */
+		if (WARN_ONCE(!port, "%s(): endpoint %s has no parent node\n",
+			      __func__, prev->full_name))
 			return NULL;
 
 		/* Avoid dropping prev node refcount to 0. */