ath9k: clean up the aggregation tid queue

Use a sk_buff_head instead containing skbs instead of a list_head
containing ath_bufs. This makes it easier to decouple the aggregation
code from the ath_buf struct

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/debug.c b/drivers/net/wireless/ath/ath9k/debug.c
index fbec5f7..727e8de 100644
--- a/drivers/net/wireless/ath/ath9k/debug.c
+++ b/drivers/net/wireless/ath/ath9k/debug.c
@@ -711,7 +711,7 @@
 					" tid: %p %s %s %i %p %p\n",
 					tid, tid->sched ? "sched" : "idle",
 					tid->paused ? "paused" : "running",
-					list_empty(&tid->buf_q),
+					skb_queue_empty(&tid->buf_q),
 					tid->an, tid->ac);
 			if (len >= size)
 				goto done;