ath9k: fix duration calculation for non-aggregated packets

When not aggregating packets, fi->framelen should be passed in as length
to calculate the duration. Before the tx path rework, ath_tx_fill_desc
was called for either one aggregate, or one single frame, with the
length of the packet or the aggregate as a parameter.
After the rework, ath_tx_sched_aggr can pass a burst of single frames to
ath_tx_fill_desc and sets len=0.
Fix broken duration calculation by overriding the length in ath_tx_fill_desc
before passing it to ath_buf_set_rate.

Cc: stable@vger.kernel.org
Reported-by: NSimon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -1276,6 +1276,10 @@ static void ath_tx_fill_desc(struct ath_softc *sc, struct ath_buf *bf,
 				if (!rts_thresh || (len > rts_thresh))
 					rts = true;
 			}
+
+			if (!aggr)
+				len = fi->framelen;
+
 			ath_buf_set_rate(sc, bf, &info, len, rts);
 		}