rtc: ds1307: ds3231 temperature s16 overflow

while retrieving temperature from ds3231, the result may be overflow
since s16 is too small for a multiplication with 250.

ie. if temp_buf[0] == 0x2d, the result (s16 temp) will be negative.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Tested-by: Michael Tatarinov <kukabu@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-ds1307.c

@@ -863,7 +863,7 @@ static void ds1307_trickle_of_init(struct i2c_client *client,
  * A user-initiated temperature conversion is not started by this function,
  * so the temperature is updated once every 64 seconds.
  */
-static int ds3231_hwmon_read_temp(struct device *dev, s16 *mC)
+static int ds3231_hwmon_read_temp(struct device *dev, s32 *mC)
 {
 	struct ds1307 *ds1307 = dev_get_drvdata(dev);
 	u8 temp_buf[2];
@@ -892,7 +892,7 @@ static ssize_t ds3231_hwmon_show_temp(struct device *dev,
 				struct device_attribute *attr, char *buf)
 {
 	int ret;
-	s16 temp;
+	s32 temp;
 
 	ret = ds3231_hwmon_read_temp(dev, &temp);
 	if (ret)