ath9k: fix clearing expired A-MPDU subframes in tx completion

When the tid aggregation state has been marked as inactive, free
completed tx packets immediately. When a new aggregation session has not
been initialized yet, the BAW checks do not recognize it as expired.

Might fix potential stalls in setting up a new aggregation session.

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