cpufreq: apple-soc: Fix an IS_ERR() vs NULL check

The of_iomap() function returns NULL if it fails.  It never returns
error pointers.  Fix the check accordingly.

Fixes: 6286bbb ("cpufreq: apple-soc: Add new driver to control Apple SoC CPU P-states")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Eric Curtin <ecurtin@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/apple-soc-cpufreq.c

@@ -189,8 +189,8 @@ static int apple_soc_cpufreq_find_cluster(struct cpufreq_policy *policy,
 	*info = match->data;
 
 	*reg_base = of_iomap(args.np, 0);
-	if (IS_ERR(*reg_base))
-		return PTR_ERR(*reg_base);
+	if (!*reg_base)
+		return -ENOMEM;
 
 	return 0;
 }