ath9k: improve ath_tx_aggr_stop to avoid TID stuck in cleanup state

When tx agg is being stopped TID is flushed using ath_tx_flush_tid. It
is possible that ath_tx_flush_tid completelly flushes TID (if all
packets in this TID have already been retried). If this happened
ath_tx_aggr_stop would leave TID in cleanup state permanently.
Fix this by making ath_tx_flush_tid remove AGGR_ADDBA_COMPLETE and
AGGR_CLEANUP flags from TID status if TID is empty.

Signed-off-by: Nikolay Martynov <mar.kolya@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>