hwmon: (adt7473) Fix voltage conversion routines

Fix voltage conversion routines. Based on an earlier patch from
Paulius Zaleckas.

According to the datasheet voltage is scaled with resistors and
value 192 is nominal voltage. 0 is 0V.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Cc: Darrick J. Wong <djwong@us.ibm.com>