ath9k: prepare queueing code for handling unaggregated traffic

- Allow ath_tx_get_tid_subframe to return non-AMPDU subframes.
- Reset the tid paused state on aggregation stop
- Initialize software queues even when HT is not supported

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