ath6kl: Add support for uAPSD

* A new APSD power save queue is added in the station structure.
* When a station has APSD capability and goes to power save, the frame
  designated to the station will be buffered in APSD queue.
* When the host receives a frame which the firmware marked as trigger,
  host delivers the buffered frame from the APSD power save queue.
  Number of frames to deliver is decided by MAX SP length.
* When a station moves from sleep to awake state, all frames buffered
  in APSD power save queue are sent to the firmware.
* When a station is disconnected, all frames bufferes in APSD power save
  queue are dropped.
* When the host queues the first frame to the APSD queue or removes the
  last frame from the APSD queue, it is indicated to the firmware using
  WMI_AP_APSD_BUFFERED_TRAFFIC_CMD.

kvalo: fix buggy handling of sks queues, made it more obvious
the user priority when wmm is disabled, remove unneed else block and
combined some variable declarations

Signed-off-by: Thirumalai Pachamuthu <tpachamu@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>