hwmon: (ds1621) Fix temperature rounding operations

Commit "hwmon: (ds1621) Add ds1721 chip support" broke rounding
for chips or configurations with less than 12 bit resolution.

Tested-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>