ath10k: use idr api for msdu_ids

HTT Tx protocol uses arbitrary host assigned ids
too associate with MSDUs when delivering
completions.

Instead of rolling out own id generation scheme
use the tools provided in kernel.

This should have little to no effect on
performance.

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