ath9k: reinitialize block ack window data when starting aggregation

There might be some old stale data left, which could confuse tracking
of pending tx frames.

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/xmit.c b/drivers/net/wireless/ath/ath9k/xmit.c
index 6ddba4b..ab4f7b4 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -858,6 +858,9 @@
 	txtid->paused = true;
 	*ssn = txtid->seq_start = txtid->seq_next;
 
+	memset(txtid->tx_buf, 0, sizeof(txtid->tx_buf));
+	txtid->baw_head = txtid->baw_tail = 0;
+
 	return 0;
 }