asus-wmi: factorise wmi_evaluate_method call

This patch create a single function to call the
WMI methods. This function handle inexistent methods (when
implemented by the WMI devices, and this is not the case on
Eee PCs), ACPI errors, etc..

Also pack struct bios_arg, and make sure that we always send
a 64bit buffer when calling a WMI method, because this is
needed on Asus notebooks.

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