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: e3cbf478f846 ("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>

target/linux/ramips/files-4.14/drivers/net/ethernet/mtk/mtk_eth_soc.c

@@ -1513,7 +1513,7 @@ static int fe_probe(struct platform_device *pdev)
 		soc->reg_table = fe_reg_table;
 
 	fe_base = devm_ioremap_resource(&pdev->dev, res);
-	if (!fe_base) {
+	if (IS_ERR(fe_base)) {
 		err = -EADDRNOTAVAIL;
 		goto err_out;
 	}