stmmac: Don't exit mdio registration when mdio subnode is not found in the DTS

Originally, most of the platforms using this driver did not define an mdio subnode
in the devicetree. Commit e34d65 ("stmmac: create of compatible mdio bus for stmmac driver")
introduced a backward compatibily issue by using of_mdiobus_register explicitly
with an mdio subnode. This patch fixes the issue by calling the function
mdiobus_register, when mdio subnode is not found. The driver is now compatible
with both modes.

Fixes: e34d6569 ("stmmac: create of compatible mdio bus for stmmac driver")
Signed-off-by: NRomain Perier <romain.perier@gmail.com>
Tested-by: NPhil Reid <preid@electromag.com.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_mdio.c

@@ -217,8 +217,7 @@ int stmmac_mdio_register(struct net_device *ndev)
 		if (mdio_node) {
 			netdev_dbg(ndev, "FOUND MDIO subnode\n");
 		} else {
-			netdev_err(ndev, "NO MDIO subnode\n");
-			return 0;
+			netdev_warn(ndev, "No MDIO subnode found\n");
 		}
 	}
 
@@ -244,7 +243,10 @@ int stmmac_mdio_register(struct net_device *ndev)
 	new_bus->phy_mask = mdio_bus_data->phy_mask;
 	new_bus->parent = priv->device;
 
-	err = of_mdiobus_register(new_bus, mdio_node);
+	if (mdio_node)
+		err = of_mdiobus_register(new_bus, mdio_node);
+	else
+		err = mdiobus_register(new_bus);
 	if (err != 0) {
 		pr_err("%s: Cannot register as MDIO bus\n", new_bus->name);
 		goto bus_register_fail;