acer-wmi: does not allow negative number set to initial device state

The driver set module parameter value: mailled, threeg and brightness
to BIOS by evaluate wmi method when driver was initialed. The default
values for those parameters are -1, so, that will be better don't set
negative value to BIOS.

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>
Signed-off-by: Lee, Chun-Yi <jlee@novell.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>