net: ethernet: mediatek: fix error handling inside mtk_mdio_init

Return -ENODEV if the MDIO bus is disabled in the device tree.
Signed-off-by: NSean Wang <sean.wang@mediatek.com>
Acked-by: NJohn Crispin <john@phrozen.org>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -304,7 +304,7 @@ static int mtk_mdio_init(struct mtk_eth *eth)
 	}
 
 	if (!of_device_is_available(mii_np)) {
-		ret = 0;
+		ret = -ENODEV;
 		goto err_put_node;
 	}