ath9k: fix ready time of the multicast buffer queue

qi->tqi_readyTime is written directly to registers that expect
microseconds as unit instead of TU.
When setting the CABQ ready time, cur_conf->beacon_interval is in TU, so
convert it to microseconds before passing it to ath9k_hw.

This should hopefully fix some Tx DMA issues with buffered multicast
frames in AP mode.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -1699,7 +1699,7 @@ int ath_cabq_update(struct ath_softc *sc)
 
 	ath9k_hw_get_txq_props(sc->sc_ah, qnum, &qi);
 
-	qi.tqi_readyTime = (cur_conf->beacon_interval *
+	qi.tqi_readyTime = (TU_TO_USEC(cur_conf->beacon_interval) *
 			    ATH_CABQ_READY_TIME) / 100;
 	ath_txq_update(sc, qnum, &qi);