hwmon: (nct6775) Fix fan speed set failure in automatic mode

Setting the fan speed is only valid in manual mode; it is not possible
to set the fan's speed in automatic mode.
Return error when attempting to set the fan speed in automatic mode.
Signed-off-by: Xing Tong Wu <xingtong.wu@siemens.com>
Link: https://lore.kernel.org/r/20231121081604.2499-3-xingtong_wu@163.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/nct6775-core.c

@@ -2553,6 +2553,13 @@ store_pwm(struct device *dev, struct device_attribute *attr, const char *buf,
 	int err;
 	u16 reg;
 
+	/*
+	 * The fan control mode should be set to manual if the user wants to adjust
+	 * the fan speed. Otherwise, it will fail to set.
+	 */
+	if (index == 0 && data->pwm_enable[nr] > manual)
+		return -EBUSY;
+
 	err = kstrtoul(buf, 10, &val);
 	if (err < 0)
 		return err;