ath_rate_sample: fix setting HT rates

ieee80211_node_set_txrate_ht_mcsrate() takes an MCS rate from 0..76,
the high bit (IEEE80211_RATE_MCS) must not be set.

This is definitely my fault - I likely didn't get to testing this
diff when I changed it from ieee80211_node_set_txrate_dot11rate()
just before landing.

Differential Revision: https://reviews.freebsd.org/D49197
Reviewed by: bz