hwmon: (nct7904) Fix incorrect range of temperature limit registers

The format of temperature limitation registers are 8-bit 2's complement
and the range is -128~127.
Converts the reading value to signed char to fix the incorrect range
of temperature limitation registers.

Signed-off-by: Amy Shih <amy.shih@advantech.com.tw>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>