acer-wmi: remove useless input argument for internal wmi

The "wmi_interface *iface" is a useless input argument for internal wmi get/set
functions, remove it to clear up source code.

Tested on Lenovo E520.
Tested on Acer TravelMate 4750.

Tested-by: mr.kobzar <mr.kobzar@gmail.com>
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>