flush kacpi_notify_wq before removing notify handler

Flush kacpi_notify_wq before notify handler is removed,
this can fix a bug which the deferred notify handler is executed
after the notify_handler has already been removed.
http://bugzilla.kernel.org/show_bug.cgi?id=9772

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>