staging: mt7621-eth: fix return value check in mtk_probe()

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

Fixes: e3cbf478 ("staging: mt7621-eth: add the drivers core files")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-eth/mtk_eth_soc.c

@@ -2077,8 +2077,8 @@ static int mtk_probe(struct platform_device *pdev)
 		return -ENOMEM;
 
 	eth->base = devm_ioremap_resource(&pdev->dev, res);
-	if (!eth->base)
-		return -EADDRNOTAVAIL;
+	if (IS_ERR(eth->base))
+		return PTR_ERR(eth->base);
 
 	spin_lock_init(&eth->page_lock);