This patch fixes the following problems with the rate control feedback
generated by ath9k for A-MPDU frames:

1. Rate control feedback is carried on the first frame of an aggregate
that is either ACKed, or has execeeded the software retry count and is
considered failed. However, ath9k would incorrectly assume the aggregate
had the length 1 if one of these conditions did not apply to the first
frame of the aggregate, but instead a later frame. This fix therefor
copies the bf_nframes field of the buffer in the same manner as the rates
field of the tx status.

2. Sometimes the ampdu_len and ampdu_ack_len fields of the tx status was
left uninitialized eventhough the IEEE80211_TX_STAT_AMPDU flag was set.
This is now avoid by setting flag and fields in the same place.

3. Even if a frame has been selected for aggregation by mac80211 and
marked with the IEEE80211_TX_CTL_AMPDU flag it can sometimes happen that
ath9k transmits the frame without aggregation. In these cases the
ampdu_ack_len field could be incorrectly computed because the nbad
parameter to ath_tx_rc_status was incorrect.

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>