hwmon: (gl518sm) Don't create sysfs files for missing features

The early revisions of the GL518SM do not report voltage values for
the first 3 voltage channels. We should not create sysfs attributes
for these missing features.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>