ath9k: fix queue depth check for forming new aggregates

To improve aggregation length, there should not be more than two fully formed
A-MPDU frames in the hardware queue. To ensure this, the code checks the tx
queue length before forming new A-MPDUs. This can reduce the throughput (or
maybe even starve out A-MPDU traffic) when too many non-aggregated frames are
in the queue.
Fix this by keeping track of pending A-MPDU frames (even when they're sent out
as single frames), but exclude rate control probing frames to improve
performance.

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/ath9k.h b/drivers/net/wireless/ath/ath9k/ath9k.h
index b0b1216..9fd9519 100644
--- a/drivers/net/wireless/ath/ath9k/ath9k.h
+++ b/drivers/net/wireless/ath/ath9k/ath9k.h
@@ -189,6 +189,7 @@
 	struct list_head axq_q;
 	spinlock_t axq_lock;
 	u32 axq_depth;
+	u32 axq_ampdu_depth;
 	bool stopped;
 	bool axq_tx_inprogress;
 	struct list_head axq_acq;