hwmon: (tmp421) fix rounding for negative values

Old code produces -24999 for 0b1110011100000000 input in standard format due to
always rounding up rather than "away from zero".

Use the common macro for division, unify and simplify the conversion code along
the way.

Fixes: 9410700b88 ("hwmon: Add driver for Texas Instruments TMP421/422/423 sensor chips")
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Link: https://lore.kernel.org/r/20210924093011.26083-3-fercerpav@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/tmp421.c

@@ -100,23 +100,17 @@ struct tmp421_data {
 	s16 temp[4];
 };
 
-static int temp_from_s16(s16 reg)
+static int temp_from_raw(u16 reg, bool extended)
 {
 	/* Mask out status bits */
 	int temp = reg & ~0xf;
 
-	return (temp * 1000 + 128) / 256;
-}
+	if (extended)
+		temp = temp - 64 * 256;
+	else
+		temp = (s16)temp;
 
-static int temp_from_u16(u16 reg)
-{
-	/* Mask out status bits */
-	int temp = reg & ~0xf;
-
-	/* Add offset for extended temperature range. */
-	temp -= 64 * 256;
-
-	return (temp * 1000 + 128) / 256;
+	return DIV_ROUND_CLOSEST(temp * 1000, 256);
 }
 
 static int tmp421_update_device(struct tmp421_data *data)
@@ -172,10 +166,8 @@ static int tmp421_read(struct device *dev, enum hwmon_sensor_types type,
 
 	switch (attr) {
 	case hwmon_temp_input:
-		if (tmp421->config & TMP421_CONFIG_RANGE)
-			*val = temp_from_u16(tmp421->temp[channel]);
-		else
-			*val = temp_from_s16(tmp421->temp[channel]);
+		*val = temp_from_raw(tmp421->temp[channel],
+				     tmp421->config & TMP421_CONFIG_RANGE);
 		return 0;
 	case hwmon_temp_fault:
 		/*