ath9k: make ath_frame_info fit into reduced-size rate_driver_data

pre-requisite to shrink size of ieee80211_tx_info which in turn is
needed to shrink skb->cb to 40 bytes again.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>