mac80211: document IEEE80211_TXCTL_OFDM_HT

This patch clarifies the use of IEEE80211_TXCTL_OFDM_HT flag.

Can by united with patch "mac80211: adding mac80211_tx_control
flags and HT flags"

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>