net: cpsw: fix null dereference at probe

Fix null-pointer dereference at probe when the mdio platform device is
missing (e.g. when it has been disabled in DT).

Cc: stable <stable@vger.kernel.org>	# v3.8
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/ti/cpsw.c

@@ -1871,6 +1871,10 @@ static int cpsw_probe_dt(struct cpsw_platform_data *data,
 		mdio_node = of_find_node_by_phandle(be32_to_cpup(parp));
 		phyid = be32_to_cpup(parp+1);
 		mdio = of_find_device_by_node(mdio_node);
+		if (!mdio) {
+			pr_err("Missing mdio platform device\n");
+			return -EINVAL;
+		}
 		snprintf(slave_data->phy_id, sizeof(slave_data->phy_id),
 			 PHY_ID_FMT, mdio->name, phyid);