[ath] fix EDMA TX buffer flags for use when retransmitting frames.

This started showing up when doing lots of aggregate traffic. For TDMA it's
always no-ACK traffic and I didn't notice this, and I didn't notice it
when doing 11abg traffic as it didn't fail enough in a bad way to trigger
this.

This showed up as the fifo depth being < 0.

Eg:

Jun 19 09:23:07 gertrude kernel: ath0: ath_tx_edma_push_staging_list: queued 2 packets; depth=2, fifo depth=1
Jun 19 09:23:07 gertrude kernel: ath0: ath_edma_tx_processq: Q1, bf=0xfffffe000385f068, start=1, end=1
Jun 19 09:23:07 gertrude kernel: ath0: ath_edma_tx_processq: Q1: FIFO depth is now 0 (1)
Jun 19 09:23:07 gertrude kernel: ath0: ath_edma_tx_processq: Q1, bf=0xfffffe0003866fe8, start=0, end=1
Jun 19 09:23:07 gertrude kernel: ath0: ath_edma_tx_processq: Q1: FIFO depth is now -1 (0)

So, clear the flags before adding them to a TX queue, so if they're
re-added for the retransmit path it'll clear whatever they were and
not double-account the FIFOEND flag.  Oops.

Tested:

* AR9380, STA mode, 11n iperf testing (~130mbit)

Approved by:	re (delphij)

sys/dev/ath/if_ath_tx_edma.c

@@ -192,6 +192,9 @@ ath_tx_edma_push_staging_list(struct ath_softc *sc, struct ath_txq *txq,
 
 		/* Queue it into our staging list */
 		TAILQ_INSERT_TAIL(&sq, bf, bf_list);
+
+		/* Ensure the flags are cleared */
+		bf->bf_flags &= ~(ATH_BUF_FIFOPTR | ATH_BUF_FIFOEND);
 		sqdepth++;
 	}