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.
Cc: stable@vger.kernel.org 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>