powercap/drivers/dtpm: Fix an IS_ERR() vs NULL check

The powercap_register_control_type() function never returns NULL, it
returns error pointers on error so update this check.

Fixes: a20d0ef9 ("powercap/drivers/dtpm: Add API for dynamic thermal power management")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/powercap/dtpm.c

@@ -467,9 +467,9 @@ static int __init dtpm_init(void)
 	struct dtpm_descr **dtpm_descr;
 
 	pct = powercap_register_control_type(NULL, "dtpm", NULL);
-	if (!pct) {
+	if (IS_ERR(pct)) {
 		pr_err("Failed to register control type\n");
-		return -EINVAL;
+		return PTR_ERR(pct);
 	}
 
 	for_each_dtpm_table(dtpm_descr)