ath9k: avoid sending a-mpdu packets to sleeping stations

If the driver gets a tx status report for an A-MPDU sent to a station that
just went to sleep, that leaves a race condition where this tx status can
trigger another A-MPDU transmission.
To fix this, check if the station is sleeping before queueing the tid.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>