ath9k: built-in rate control A-MPDU fix

This patch attempts to ensure that ath9k's built-in rate control algorithm
does not rely on the value of the ampdu_len and ampdu_ack_len tx status
fields unless the IEEE80211_TX_STAT_AMPDU flag is set.

This patch has not been tested.

Cc: <stable@kernel.org>
Signed-off-by: Björn Smedman <bjorn.smedman@venatech.se>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>