acer-wmi: Remove private workqueue

As per Dmitry Torokhov's suggestion, acer-wmi doesn't need a private
workqueue, so remove it.

Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>