asus-wmi: update wlan LED through rfkill led trigger

For those machines with wapf=4, BIOS won't update the wireless LED,
since wapf=4 means user application will take in chage of the wifi and bt.
So, we have to update wlan LED status explicitly.

But I found there is another wireless LED bug in launchpad and which is
not in the wapf=4 quirk.
So, it might be better to set wireless LED status explicitly for all
machines.

BugLink: https://launchpad.net/bugs/901105



Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>