ath9k: clean up processing of pending tx frames on reset

Dropping packets from aggregation sessions is usually not a good idea, as
it might upset the synchronization of the BlockAck receive window of the
remote node. The use of the retry_tx parameter to reset/tx-drain functions
also seemed a bit arbitrary.
This patch removes this parameter altogether and ensures that pending tx
frames are not dropped for no good reason.

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