asus-wmi: add calls to INIT, SPEC and SFUN on init

INIT() call is needed to enable hotkeys on G73
SPEC() and SFUN() allow us to know more about
available features.

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