hwmon/f71882fg: Break out test for auto pwm's controlled by digital readings

Putting this check in its own switch case rather then in the switch case
for adding pwm auto point sysfs attr is cleaner. This is a preparation patch
for adding support for more different models.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>