ath9k: resume aggregation immediately after a hardware reset

Since aggregation is usually triggered by tx completion, a hardware
reset (because of beacon stuck, tx hang or baseband hang) can
significantly delay the transmission of the next AMPDU (until the next
tx completion event).
Fix this by rescheduling aggregation after such a reset.

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