platform/x86: asus-wmi: Refactor charge_threshold_store()

There are few issues with the current code:
 - the error code from kstrtouint() is shadowed
 - the error code from asus_wmi_set_devstate() is ignored
 - the extra check against 0 for count (this is guaranteed by sysfs)

Fix these issues by doing a slight refactoring of charge_threshold_store().
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/asus-wmi.c

@@ -2086,13 +2086,15 @@ static ssize_t charge_threshold_store(struct device *dev,
 	int value, ret, rv;
 
 	ret = kstrtouint(buf, 10, &value);
+	if (ret)
+		return ret;
 
-	if (!count || ret != 0)
-		return -EINVAL;
 	if (value < 0 || value > 100)
 		return -EINVAL;
 
-	asus_wmi_set_devstate(ASUS_WMI_CHARGE_THRESHOLD, value, &rv);
+	ret = asus_wmi_set_devstate(ASUS_WMI_CHARGE_THRESHOLD, value, &rv);
+	if (!ret)
+		return ret;
 
 	if (rv != 1)
 		return -EIO;