iwl3945: use cancel_delayed_work_sync to cancel rfkill_poll

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=30601

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reported-by: TJ <ubuntu@tjworld.net>
Reported-by: Huaxu Wan <huaxu.wan@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>