ath9k: Fix handling of retry count of NO_ACK frames

Check for IEEE80211_TX_CTL_NO_ACK instead of is_multicast_ether_addr
when determining whether to use lowest rate without retries.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>