ramips: 5.4: handle ERR_PTR properly

of_get_mac_address can return ERR_PTR since 5.2, so the return pointer should be
checked before used. Otherwise it might cause an oops during boot.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ramips/files/drivers/net/ethernet/ralink/mtk_eth_soc.c

@@ -1401,7 +1401,7 @@ static int __init fe_init(struct net_device *dev)
 	fe_reset_phy(priv);
 
 	mac_addr = of_get_mac_address(priv->dev->of_node);
-	if (mac_addr)
+	if (!IS_ERR_OR_NULL(mac_addr))
 		ether_addr_copy(dev->dev_addr, mac_addr);
 
 	/* If the mac address is invalid, use random mac address  */