ath9k: Dont update rate control for every AMPDU

Update the rate control only with the tx status of first
AMPDU of an aggregation. This patch fixes frequent drops
in throughput.

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