hwmon/f71882fg: Make the decision wether to register fan attr. per fan

Before this patch the f71882fg driver completely fails to initialize
on systems which have reserved settings in the pwm enable register, and
it disables all auto pwm sysfs attributes if any fan is controlled by
a digital sensor reading.

This patch changes the fail to initialize into don't register any attributes
for the fan for which there are reserved settings in the pwm enable register
and also makes the not registering of auto pwm sysfs attributes a per fan
thing.

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