hwmon: (adt7475) Rework voltage inputs handling

Rework the handling of voltage inputs to make it possible and easy to
support more inputs. This will be needed for the upcoming ADT7490
support.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>