ath9k_htc: set IEEE80211_TX_STAT_AMPDU for acked aggregated frames

Frame aggregation requires the IEEE80211_TX_STAT_AMPDU to
be set so that mac80211 can report the last_tx_rate correctly.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>