Store away the RTS aggregate limit from the HAL.

This will be used by some upcoming code to ensure that aggregates
are enforced to be a certain size.  The AR5416 has a limitation on
RTS protected aggregates (8KiB).

sys/dev/ath/if_ath.c

@@ -732,6 +732,13 @@ ath_attach(u_int16_t devid, struct ath_softc *sc)
 		ic->ic_txstream = txs;
 		ic->ic_rxstream = rxs;
 
+		(void) ath_hal_getcapability(ah, HAL_CAP_RTS_AGGR_LIMIT, 1,
+		    &sc->sc_rts_aggr_limit);
+		if (sc->sc_rts_aggr_limit != (64 * 1024))
+			device_printf(sc->sc_dev,
+			    "[HT] RTS aggregates limited to %d KiB\n",
+			    sc->sc_rts_aggr_limit / 1024);
+
 		device_printf(sc->sc_dev,
 		    "[HT] %d RX streams; %d TX streams\n", rxs, txs);
 	}

sys/dev/ath/if_athvar.h

@@ -533,6 +533,7 @@ struct ath_softc {
 	uint16_t		*sc_eepromdata;	/* Local eeprom data, if AR9100 */
 	int			sc_txchainmask;	/* currently configured TX chainmask */
 	int			sc_rxchainmask;	/* currently configured RX chainmask */
+	int			sc_rts_aggr_limit;	/* TX limit on RTS aggregates */
 
 	/* Queue limits */