phy: phy-ocelot-serdes: fix return value check in serdes_probe()

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

Fixes: 51f6b410 ("phy: add driver for Microsemi Ocelot SerDes muxing")
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: NQuentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/phy/mscc/phy-ocelot-serdes.c

@@ -257,8 +257,8 @@ static int serdes_probe(struct platform_device *pdev)
 
 	ctrl->dev = &pdev->dev;
 	ctrl->regs = syscon_node_to_regmap(pdev->dev.parent->of_node);
-	if (!ctrl->regs)
-		return -ENODEV;
+	if (IS_ERR(ctrl->regs))
+		return PTR_ERR(ctrl->regs);
 
 	for (i = 0; i <= SERDES_MAX; i++) {
 		ret = serdes_phy_create(ctrl, i, &ctrl->phys[i]);