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>