hwmon: (it87) Properly handle wrong sensor type requests

Currently, if someone tries to set the thermal sensor type to an
unsupported value, subsequent accesses to the chip may temporarily
show the sensor in question as disabled. Use a temporary variable
and only update the cached value on success, to prevent such
confusion.
Signed-off-by: Jean Delvare <khali@linux-fr.org>

drivers/hwmon/it87.c

@@ -539,15 +539,14 @@ static ssize_t set_sensor(struct device *dev, struct device_attribute *attr,
 
 	struct it87_data *data = dev_get_drvdata(dev);
 	long val;
+	u8 reg;
 
 	if (strict_strtol(buf, 10, &val) < 0)
 		return -EINVAL;
 
-	mutex_lock(&data->update_lock);
-
-	data->sensor = it87_read_value(data, IT87_REG_TEMP_ENABLE);
-	data->sensor &= ~(1 << nr);
-	data->sensor &= ~(8 << nr);
+	reg = it87_read_value(data, IT87_REG_TEMP_ENABLE);
+	reg &= ~(1 << nr);
+	reg &= ~(8 << nr);
 	if (val == 2) {	/* backwards compatibility */
 		dev_warn(dev, "Sensor type 2 is deprecated, please use 4 "
 			 "instead\n");
@@ -555,13 +554,14 @@ static ssize_t set_sensor(struct device *dev, struct device_attribute *attr,
 	}
 	/* 3 = thermal diode; 4 = thermistor; 0 = disabled */
 	if (val == 3)
-		data->sensor |= 1 << nr;
+		reg |= 1 << nr;
 	else if (val == 4)
-		data->sensor |= 8 << nr;
-	else if (val != 0) {
-		mutex_unlock(&data->update_lock);
+		reg |= 8 << nr;
+	else if (val != 0)
 		return -EINVAL;
-	}
+
+	mutex_lock(&data->update_lock);
+	data->sensor = reg;
 	it87_write_value(data, IT87_REG_TEMP_ENABLE, data->sensor);
 	mutex_unlock(&data->update_lock);
 	return count;