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>

drivers/net/wireless/iwlwifi/iwl3945-base.c

@@ -5196,7 +5196,7 @@ static void __devexit iwl3945_pci_remove(struct pci_dev *pdev)
 	sysfs_remove_group(&pdev->dev.kobj, &iwl3945_attribute_group);
 
 	iwl_rfkill_unregister(priv);
-	cancel_delayed_work(&priv->rfkill_poll);
+	cancel_delayed_work_sync(&priv->rfkill_poll);
 
 	iwl3945_dealloc_ucode_pci(priv);