ath9k: Fix rate control update for aggregated frames

We will miss rate control update if first A-MPDU of an
aggregation is not Block Acked as we always tell if the
rate control needs to updated through update_rc of first
A-MPDU. This patch does rate control update for the first
A-MPDU which notifies it's tx status (which is not
necessarily the first A-MPDU of an aggregation) to mac80211.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>