ath9k: Fix no-ack frame status

Check if the frame has been completed without any
error and use IEEE80211_TX_STAT_NOACK_TRANSMITTED to
indicate successful transmission of no-ack frames.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>