hwmon: (max1668) Fix writing the minimum temperature

When trying to set the minimum temperature, the driver was erroneously
writing the maximum temperature into the chip.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org # v3.2+
Reviewed-by: Jean Delvare <jdelvare@suse.de>