acer-wmi: use pr_<level> for messages

Cc: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: Corentin Chary <corentincj@iksaif.net>
Cc: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Lee, Chun-Yi <jlee@novell.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>