reset: mchp: sparx5: fix return value check in mchp_sparx5_map_io()

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

Fixes: 453ed4283beb ("reset: mchp: sparx5: add switch reset driver")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Wei Yongjun <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Philipp Zabel <[e-mail]>

drivers/reset/reset-microchip-sparx5.c

···
 	void __iomem *mem;
 
 	mem = devm_platform_get_and_ioremap_resource(pdev, index, &res);
-	if (!mem) {
+	if (IS_ERR(mem)) {
 		dev_err(&pdev->dev, "Could not map resource %d\n", index);
-		return -ENXIO;
+		return PTR_ERR(mem);
 	}
 	sparx5_reset_regmap_config.name = res->name;
 	map = devm_regmap_init_mmio(&pdev->dev, mem, &sparx5_reset_regmap_config);