athn(4): use fixed 11n Tx retry rate while probing

[Stefan Sperling]

This makes athn(4) use a constant Tx retry rate while MiRA is probing.
Should result in better initial Tx rate selection and thus less retries.

When testing this make sure your tree is up to date; my iwm(4) Tx retry
commit from today is required to compile this diff because that commit
added the new MiRA function being called here.

ok?

diff 66e914ccac1d2401a637a12167cad113075d2af0 /usr/src
blob - f58d36a19cad8454504d0b1f54ad89aa7966c822
file + sys/dev/ic/ar5008.c
--- sys/dev/ic/ar5008.c
+++ sys/dev/ic/ar5008.c
@@ -1375,6 +1375,11 @@ ar5008_tx(struct athn_softc *sc, struct mbuf *m, struc
                /* Use same fixed rate for all tries. */
                ridx[0] = ridx[1] = ridx[2] = ridx[3] =
                    sc->fixed_ridx;
+       } else if ((ni->ni_flags & IEEE80211_NODE_HT) &&
+           ieee80211_mira_is_probing(&an->mn)) {
+               /* Use same fixed rate for all tries. */
+               ridx[0] = ridx[1] = ridx[2] = ridx[3] =
+                   ATHN_RIDX_MCS0 + ni->ni_txmcs;
        } else {
                /* Use fallback table of the node. */
                int txrate;