acer-wmi: does not poll device status when WMI event is available

Acer WMI hotkey event's result include current device status, just
need sync the status to killswitch after acer-wmi driver receive
hotkey event but not always poll device status. This is good for
performance.

But, if use EC raw mode, Acer BIOS will not emit wmi event and
leave EC to control device status. So, still startup polling job
when doesn't detect WMI event GUID or user choice to use ec_raw_mode.

Tested on Acer TravelMate 8572 notebook.

Cc: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: Dmitry Torokhov <dtor@mail.ru>
Cc: Corentin Chary <corentincj@iksaif.net>
Cc: Thomas Renninger <trenn@suse.de>
Signed-off-by: Lee, Chun-Yi <jlee@novell.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>