ath9k: fix aggregation stop/flush handling

When aggregation stop is requested, don't run the mac80211 aggregation
stop callback yet, while the session is still blocked.
Also, when aggregation flush is requested, don't run the callback at all.

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