eeepc-wmi: never load if legacy device is enabled

If legacy device (SB.ATKD - ASUS010) used by eeepc-laptop
is enabled, don't allow eeepc-wmi to load because:
- eeepc-laptop may be loaded, and can conflict with
  eeepc-wmi (they both try to register eeepc::touchpad
  led for example).
- the WMI interface is inteded to be used when the OS is
  not detected as Win 7. And when this is the case, the
  ASUS010 device is disabled.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Matthew Garrett <mjg@redhat.com>