acer-wmi: Detect communication hot key number

Currently we set a fixed hot key number to 0x01 for communction button,
but, actually, the key number is different on each acer laptop and it was
reported by SMBIOS.

So, add this patch to get the communication hot key number from Acer
OEM-specific SMBIOS Type AA.

Tested on Acer TravelMate 4750

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 <mjg@redhat.com>