ath10k: refactor tx pending management

Tx pending counter logic assumed that the sk_buff
is already known and hence was performed in HTT
functions themselves.

However, for the sake of future wake_tx_queue()
usage the driver must be able to tell whether it
can submit more frames to firmware before it
dequeues frame from ieee80211_txq (and thus long
before HTT Tx functions are called) because once a
frame is dequeued it cannot be requeud back to
mac80211.

This prepares the driver for future changes.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>