hwmon: (w83627ehf) Skip reading unused voltage registers

When in6 is missing, don't read the corresponding registers, it's a
waste of time. The logic is similar to what we do for fans and
temperatures.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>