hwmon: (f71882fg) Fix negative temperature

All temperature of Fintek superio hwmonitor that using 1-byte reg will use
2's complement.

In show_temp()
	temp = data->temp[nr] * 1000;

When data->temp[nr] read as 255, it indicate -1C, but this code will report
255C to userspace. It'll be ok when change to:
	temp = ((s8)data->temp[nr]) * 1000;

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
Link: https://lore.kernel.org/r/20220418090706.6339-1-hpeter+linux_kernel@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: hpeter

drivers/hwmon/f71882fg.c

@@ -1578,8 +1578,9 @@ static ssize_t show_temp(struct device *dev, struct device_attribute *devattr,
 		temp *= 125;
 		if (sign)
 			temp -= 128000;
-	} else
-		temp = data->temp[nr] * 1000;
+	} else {
+		temp = ((s8)data->temp[nr]) * 1000;
+	}
 
 	return sprintf(buf, "%d\n", temp);
 }