acer-wmi: check wireless capability flag before register rfkill

There will be better to check the wireless capability flag
(ACER_CAP_WIRELESS) before register wireless rfkill because maybe
the machine doesn't have wifi module or the module removed by user.

Tested on Acer Travelmate 8572
Tested on Acer Aspire 4739Z

Tested-by: AceLan Kao <acelan.kao@canonical.com>
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>