net: ag71xx: fix mdio subnode support

This patch is syncing driver with actual devicetree documentation:
Documentation/devicetree/bindings/net/qca,ar71xx.txt
|Optional subnodes:
|- mdio : specifies the mdio bus, used as a container for phy nodes
|  according to phy.txt in the same directory

The driver was working with fixed phy without any noticeable issues. This bug
was uncovered by introducing dsa ar9331-switch driver.
Since no one reported this bug until now, I assume no body is using it
and this patch should not brake existing system.

Fixes: d51b6ce4 ("net: ethernet: add ag71xx driver")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/atheros/ag71xx.c

@@ -526,7 +526,7 @@ static int ag71xx_mdio_probe(struct ag71xx *ag)
 	struct device *dev = &ag->pdev->dev;
 	struct net_device *ndev = ag->ndev;
 	static struct mii_bus *mii_bus;
-	struct device_node *np;
+	struct device_node *np, *mnp;
 	int err;
 
 	np = dev->of_node;
@@ -571,7 +571,9 @@ static int ag71xx_mdio_probe(struct ag71xx *ag)
 		msleep(200);
 	}
 
-	err = of_mdiobus_register(mii_bus, np);
+	mnp = of_get_child_by_name(np, "mdio");
+	err = of_mdiobus_register(mii_bus, mnp);
+	of_node_put(mnp);
 	if (err)
 		goto mdio_err_put_clk;