acer-wmi: change to emit touchpad on off key

KEY_TOUCHPAD_TOOGLE key is for notice userland change touchpad state
via xf86-input-synaptics on the machine that don't toggle touchpad in
hardware. But, acer laptop actually toggle touchpad in hardware.
So, this patch change to emit KEY_TOUCHPAD_ON/OFF key when acer-wmi grab
device state of touchpad.

Reference: brc#848270
        https://bugzilla.redhat.com/show_bug.cgi?id=848270



Tested-by: Nathanael Noblet <nathanael@gnat.ca>
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 <matthew.garrett@nebula.com>