[ath] fix comments!

I keep asking myself "what do these fields mean" and so now I've clarified
it for myself.

Tested:

* Reading the comments, going "a-ha!" a couple times.

Approved by:	re (gjb)

sys/dev/ath/if_athvar.h

@@ -370,9 +370,9 @@ struct ath_txq {
 	 */
 	struct {
 		TAILQ_HEAD(axq_q_f_s, ath_buf)	axq_q;
-		u_int				axq_depth;
+		u_int				axq_depth;	/* how many frames (1 per legacy, 1 per A-MPDU list) are in the FIFO queue */
 	} fifo;
-	u_int			axq_fifo_depth;	/* depth of FIFO frames */
+	u_int			axq_fifo_depth;	/* how many FIFO slots are active */
 
 	/*
 	 * XXX the holdingbf field is protected by the TXBUF lock