Kick tx after processing rx'd frames; this fixes latency issues

for processing frames from the power save queue when operating
in ap mode.  This is especially noticeable for realtime data going
to devices like voip phones.

Submitted by:	"J.R. Oldroyd" <jr@opal.com>
MFC after:	2 weeks

sys/dev/ath/if_ath.c

@@ -3170,6 +3170,11 @@ rx_next:
 	if (ngood)
 		sc->sc_lastrx = tsf;
 
+	/* NB: may want to check mgtq too */
+	if ((ifp->if_drv_flags & IFF_DRV_OACTIVE) == 0 &&
+	    !IFQ_IS_EMPTY(&ifp->if_snd))
+		ath_start(ifp);
+
 	NET_UNLOCK_GIANT();		/* XXX */
 #undef PA2DESC
 }