asus-wmi: introduce struct asus_rfkill

First, this allow use to remove the custom asusrfkill_wlan_query,
but this will also allow us to give struct asus_wmi * to
get_devstate/set_devstate later.

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