ath6kl: assign Tx packet drop threshold per endpoint on htc pipe layer

On the htc mbox layer, it will assign each endpoint (AC) with a
different Tx-packet-drop threshold, so lower priority AC is more
likely to drop packets and the cookies become more available to higher
priority AC.

On the htc pipe layer, assign the tx packet drop threshold as well, it
will let AC to drop packets when cookies below the tx packet drop
threshold.

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