hwmon/f71882fg: Add a f71882fg_create_fan_sysfs_files helper function

This is a preparation patch for not registering fan/pwm attributes for
some fans (rather then register them for all or for none).

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