net: ethernet: mtk_eth_soc: fix return value check in mtk_wed_add_hw()

If syscon_regmap_lookup_by_phandle() fails, it never return NULL pointer,
change the check to IS_ERR().

Fixes: 804775df ("net: ethernet: mtk_eth_soc: add support for Wireless Ethernet Dispatch (WED)")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_wed.c

@@ -813,7 +813,7 @@ void mtk_wed_add_hw(struct device_node *np, struct mtk_eth *eth,
 		return;
 
 	regs = syscon_regmap_lookup_by_phandle(np, NULL);
-	if (!regs)
+	if (IS_ERR(regs))
 		return;
 
 	rcu_assign_pointer(mtk_soc_wed_ops, &wed_ops);