hwmon: (it87) Simplify reading voltage registers

Voltage registers are non-sequential. Use a register array instead
of a macro to map sensor index to register to simplify the code
and to make it easier to add additional voltage sensors.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>