ath9k: fix an aggregation start related race condition

ath9k: fix an aggregation start related race condition

commit 231c3a1f0630c07a584905507a1cb7b705a56ab7 upstream.

A new aggregation session start can be issued by mac80211, even when the
cleanup of the previous session has not completed yet. Since the data structure
for the session is not recreated, this could corrupt the block ack window
and lock up the aggregation session. Fix this by delaying the new session
until the old one has been cleaned up.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>