ath9k: add sampling methods for (tx|rx) timestamp

Add sampling methods for ACK RX timestamp in ath_rx_tasklet() and for TX frame
timestamp in ath_tx_complete_aggr() and in ath_tx_process_buffer(). These
samples will be used in dynack processing for ACK timeout estimation

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
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 2819866..93ad31b 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -587,6 +587,10 @@
 				memcpy(tx_info->control.rates, rates, sizeof(rates));
 				ath_tx_rc_status(sc, bf, ts, nframes, nbad, txok);
 				rc_update = false;
+				if (bf == bf->bf_lastbf)
+					ath_dynack_sample_tx_ts(sc->sc_ah,
+								bf->bf_mpdu,
+								ts);
 			}
 
 			ath_tx_complete_buf(sc, bf, txq, &bf_head, ts,
@@ -687,6 +691,7 @@
 			memcpy(info->control.rates, bf->rates,
 			       sizeof(info->control.rates));
 			ath_tx_rc_status(sc, bf, ts, 1, txok ? 0 : 1, txok);
+			ath_dynack_sample_tx_ts(sc->sc_ah, bf->bf_mpdu, ts);
 		}
 		ath_tx_complete_buf(sc, bf, txq, bf_head, ts, txok);
 	} else