clocksource/drivers/ingenic_ost: Fix return value check in ingenic_ost_probe()

In case of error, the function device_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: ca7b72b5a5f2 ("clocksource: Add driver for the Ingenic JZ47xx OST")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Wei Yongjun <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/clocksource/ingenic-ost.c

···
 		return PTR_ERR(ost->regs);
 
 	map = device_node_to_regmap(dev->parent->of_node);
-	if (!map) {
+	if (IS_ERR(map)) {
 		dev_err(dev, "regmap not found");
-		return -EINVAL;
+		return PTR_ERR(map);
 	}
 
 	ost->clk = devm_clk_get(dev, "ost");