acer-wmi: check the existence of internal wireless device when set capability

That will be better to check the existence of internal wireless device
when we set wireless capability and generate killswitch for it. It can
avoid userland access wireless rfkill but the machine doesn't have internal
wireless device.

Tested on Acer Travelmate 8572

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@suse.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>