sony-laptop: generalise ACPI calls into SNC functions

All calls into the SNC device methods have zero or one arguments and
return an integer or a buffer (some functions go as far as returning an
integer _or_ a buffer depending on the input parameter...).
This allows simplifying a couple of code paths and prepares the field
for other users of functions returning buffers.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <mjg@redhat.com>