dell-wmi: do not keep driver loaded on unsupported boxes

There is no point in having the driver loaded in memory if we fail
to locate particular WMI GUID.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>