ath6kl: handle background(BK) stream properly on htc mbox layer

When a STA sends huge BK QoS data frame first and then sends BE/VI/VO
QoS data frame, the corresponding throughput becomes much lower than
that without sends BK QoS data frame before.

The root cause is that when station send BK stream, the tx credits of
BK stream don't return back to higher priority QoS stream such as BE,
VI, and VO stream. This patch will handle BK stream properly, when
there is higher priority QoS stream, it will seek tx credits from BK
stream properly.

Signed-off-by: Kevin Fang <kevin.fang@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>