platform/x86: dell-wmi-sysman: Fix class device unregistration

platform/x86: dell-wmi-sysman: Fix class device unregistration

Devices under the firmware_attributes_class do not have unique a dev_t.
Therefore, device_unregister() should be used instead of
device_destroy(), since the latter may match any device with a given
dev_t.

Fixes: e8a60aa7404b ("platform/x86: Introduce support for Systems Management Driver over WMI for Dell Systems")
Signed-off-by: Kurt Borja <kuurtb@gmail.com>
Link: https://lore.kernel.org/r/20250625-dest-fix-v1-3-3a0f342312bb@gmail.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>