[ath] initialise do_ldpc to 0.

I .. can't believe I missed this.

This showed up because the AP was TX'ing LDPC to an iwm(4) chipset,
which didn't advertise LDPC and doesn't /accept/ LDPC.  Amusingly, all
the two other FreeBSD 11n parts I had tested with (AR9380, Intel 7260)
and I completely forgot to test on ye olde hardware.

That'll teach me.

Tested:

* AR9580 (AP) - Intel 7260 (STA), AR9380 (STA), Intel 6205 (STA)

sys/dev/ath/if_ath_tx_ht.c

@@ -237,6 +237,7 @@ ath_tx_rate_fill_rcflags(struct ath_softc *sc, struct ath_buf *bf)
 	 * It's a global flag, not a per-try flag, so we clear
 	 * it if any of the rate entries aren't 11n.
 	 */
+	do_ldpc = 0;
 	if ((ni->ni_vap->iv_htcaps & IEEE80211_HTCAP_LDPC) &&
 	    (ni->ni_htcap & IEEE80211_HTCAP_LDPC))
 		do_ldpc = 1;