mac80211: fix sta_info kernel-doc warning

Sorry, forgot to run kernel-doc and just got the output from the nightly
run by email, this fixes a warning which I introduced when doing the
first RC API cleanups.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/sta_info.h

@@ -197,7 +197,7 @@ struct sta_ampdu_mlme {
  * @tx_packets: number of RX/TX MSDUs
  * @tx_bytes: TBD
  * @tx_fragments: number of transmitted MPDUs
- * @txrate_idx: TBD
+ * @last_txrate_idx: Index of the last used transmit rate
  * @tid_seq: TBD
  * @wme_tx_queue: TBD
  * @ampdu_mlme: TBD