hwmon: (it87) Save fan registers in 2-dimensional array

Also unify fan functions to use the same code for 8 and 16 bit fans.

This patch reduces code size by approximately 1,200 bytes on x86_64.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>