hwmon: (emc6w201) Fix DIV_ROUND_CLOSEST problem with unsigned divisors

Result of DIV_ROUND_CLOSEST is undefined for negative dividends if the divisor
variable type is unsigned. Fix by declaring divisor as signed variable.

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