ath9k: remove useless flag conversation.

some flags used only outside of ath9k - In this case we can use
"enum mac80211_rx_flags" and pass it upstream without extra
conversation.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>