ath9k: fix aggregation session lockup

ath9k: fix aggregation session lockup

commit c01fac1c77a00227f706a1654317023e3f4ac7f0 upstream.

If an aggregation session fails, frames still end up in the driver queue
with IEEE80211_TX_CTL_AMPDU set.
This causes tx for the affected station/tid to stall, since
ath_tx_get_tid_subframe returning packets to send.

Fix this by clearing IEEE80211_TX_CTL_AMPDU as long as no aggregation
session is running.

Reported-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>