mac80211: add missing IEEE80211_HW_SUPPORTS_HT_CCK_RATES docs

Document the IEEE80211_HW_SUPPORTS_HT_CCK_RATES flag.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -1503,6 +1503,10 @@ struct ieee80211_tx_control {
  * @IEEE80211_HW_TIMING_BEACON_ONLY: Use sync timing from beacon frames
  *	only, to allow getting TBTT of a DTIM beacon.
  *
+ * @IEEE80211_HW_SUPPORTS_HT_CCK_RATES: Hardware supports mixing HT/CCK rates
+ *	and can cope with CCK rates in an aggregation session (e.g. by not
+ *	using aggregation for such frames.)
+ *
  * @IEEE80211_HW_CHANCTX_STA_CSA: Support 802.11h based channel-switch (CSA)
  *	for a single active channel while using channel contexts. When support
  *	is not enabled the default action is to disconnect when getting the