V4L/DVB (7320): pvrusb2: Eliminate timer race during tear-down

The pvrusb2 tear-down logic was clearing two timers before stopping
its internal work queue.  That left a tiny window open where the work
queue might run after the timers are stopped, possibly starting them
again.  This could lead to dangling pointers and an oops.  Solution:
Kill the work queue first, then delete the timers.
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/pvrusb2/pvrusb2-hdw.c

@@ -2114,13 +2114,13 @@ void pvr2_hdw_destroy(struct pvr2_hdw *hdw)
 {
 	if (!hdw) return;
 	pvr2_trace(PVR2_TRACE_INIT,"pvr2_hdw_destroy: hdw=%p",hdw);
-	del_timer_sync(&hdw->quiescent_timer);
-	del_timer_sync(&hdw->encoder_wait_timer);
 	if (hdw->workqueue) {
 		flush_workqueue(hdw->workqueue);
 		destroy_workqueue(hdw->workqueue);
 		hdw->workqueue = NULL;
 	}
+	del_timer_sync(&hdw->quiescent_timer);
+	del_timer_sync(&hdw->encoder_wait_timer);
 	if (hdw->fw_buffer) {
 		kfree(hdw->fw_buffer);
 		hdw->fw_buffer = NULL;