ath10k: fix fw stats processing

If stat data exceeds wmi-htc buffer limits
firmware splits it into many wmi stats update
events which are delivered in a ping-pong fashion
triggered by wmi stats request command.

Since there's only an implicit start-of-data and
no end-of-data indications the driver has to
perform some trickery to get complete stat data.

kvalo: use %zu to fix a compiler warning and fix a typo in a comment

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