hwmon: (emc6w201) Fix temperature limit range

Temperature limit range is [-127, 127], not [-127, 128].
The wrong range caused a bad limit to be written into the chip
if the limit was set to a value of 128 degrees C or above.

Also use DIV_ROUND_CLOSEST instead of a plain divide operation
to reduce the rounding error when writing temperature limits.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>