ath10k: add HTC TX credits replenishing notification

This will allow higher layers to anticipate and
act upon TX credits renewal. This will be
important for some future rework of WMI command
submission.

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