ath9k: Handle mac80211's RC flags for MCS rates

mac80211 notifies the RC algorithm if RTS/CTS and short preamble
are needed. The RC flags for MCS rates are currently not handled
by mac80211, and ath9k's RC doesn't set the flags either. Fix this.

Also, set the rts_cts_rate_idx inside the RC algorithm.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>