i2o: destroy event queue only when drv->event is set

i2o_driver_register() initalizes event queue for driver only when
drv->event is set.  So similarly the event queue should be destroyed only
when drv->event is set in the error path.  Otherwise destroy_workqueue()
will called with NULL.

Cc: Markus Lidel <Markus.Lidel@shadowconnect.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/message/i2o/driver.c

@@ -123,8 +123,12 @@ int i2o_driver_register(struct i2o_driver *drv)
 	}
 
 	rc = driver_register(&drv->driver);
-	if (rc)
-		destroy_workqueue(drv->event_queue);
+	if (rc) {
+		if (drv->event) {
+			destroy_workqueue(drv->event_queue);
+			drv->event_queue = NULL;
+		}
+	}
 
 	return rc;
 };