hwmon: (gpio-fan) Switch to using the new API kobj_to_dev()

fixed the following coccicheck:
drivers/hwmon/gpio-fan.c:302:60-61: WARNING opportunity for
kobj_to_dev()
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1609211837-54930-1-git-send-email-tiantao6@hisilicon.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/gpio-fan.c

@@ -299,7 +299,7 @@ static DEVICE_ATTR(fan1_target, 0644, fan1_input_show, set_rpm);
 static umode_t gpio_fan_is_visible(struct kobject *kobj,
 				   struct attribute *attr, int index)
 {
-	struct device *dev = container_of(kobj, struct device, kobj);
+	struct device *dev = kobj_to_dev(kobj);
 	struct gpio_fan_data *data = dev_get_drvdata(dev);
 
 	if (index == 0 && !data->alarm_gpio)