hwmon: (adm1029) Use mask for fan_div value

This is my patch for testing correct values of fan div in adm1029 and
prevent a division by 0 for some (unlikely) register values.

Signed-off-by: Corentin Labbe <corentin.labbe@geomatys.fr>
Signed-off-by: Jean Delvare <khali@linux-fr.org>