ag71xx: fix return value check in ag71xx_probe()

In case of error, the function of_get_mac_address() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: d51b6ce ("net: ethernet: add ag71xx driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/atheros/ag71xx.c

@@ -1735,9 +1735,9 @@ static int ag71xx_probe(struct platform_device *pdev)
 	ag->stop_desc->next = (u32)ag->stop_desc_dma;
 
 	mac_addr = of_get_mac_address(np);
-	if (mac_addr)
+	if (!IS_ERR(mac_addr))
 		memcpy(ndev->dev_addr, mac_addr, ETH_ALEN);
-	if (!mac_addr || !is_valid_ether_addr(ndev->dev_addr)) {
+	if (IS_ERR(mac_addr) || !is_valid_ether_addr(ndev->dev_addr)) {
 		netif_err(ag, probe, ndev, "invalid MAC address, using random address\n");
 		eth_random_addr(ndev->dev_addr);
 	}