hwmon: (w83791d) Use fan divisor bits from vbat register

Update w83791d with fan bits in vbat mon register (7.48 of the
datasheet). This change allows all fans to have a divisor of 128, 
and fixes a problem with incorrectly reported fan speeds. 

Signed-off-by: Marc Hulsman <m.hulsman@tudelft.nl>
Signed-off-by: Jean Delvare <khali@linux-fr.org>