Users reported lockup with work still trying to run
after module has been unloaded.

http://thread.gmane.org/gmane.linux.kernel.wireless.general/30594/focus=30601Signed-off-by: NReinette Chatre <reinette.chatre@intel.com>
Reported-by: NTJ <ubuntu@tjworld.net>
Reported-by: NHuaxu Wan <huaxu.wan@linux.intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>