ath5k: fix RTS/CTS by using proper rate flags

The rates in the tx control rateset do not have the protection
flags applied, so RTS/CTS would never get enabled if requested.

Fix by using the rate flags in the rates returned by
ieee80211_get_tx_rates().

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>