mac80211: expose txq queue depth and size to drivers

This will allow drivers to make more educated
decisions whether to defer transmission or not.

Relying on wake_tx_queue() call count implicitly
was not possible because it could be called
without queued frame count actually changing on
software tx aggregation start/stop code paths.

It was also not possible to know how long
byte-wise queue was without dequeueing.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.c

@@ -116,6 +116,7 @@ static void __cleanup_single_sta(struct sta_info *sta)
 
 			ieee80211_purge_tx_queue(&local->hw, &txqi->queue);
 			atomic_sub(n, &sdata->txqs_len[txqi->txq.ac]);
+			txqi->byte_cnt = 0;
 		}
 	}