platform/x86: dell-wmi-sysman: Use firmware_attributes_class helper

Update Dell WMI sysman driver to use newly implemented helper module.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mark Pearson <markpearson@lenovo.com>
Link: https://lore.kernel.org/r/20210530223111.25929-2-markpearson@lenovo.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>