hwmon: (adm1031) Allow setting update rate

Based on earlier work by Ira W. Snyder.

The adm1031 chip is capable of using a runtime configurable sampling rate,
using the fan filter register. Add support for reading and setting the
update rate via sysfs.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Ira W. Snyder <iws@ovro.caltech.edu>