ath10k: use rx descriptor for ppdu status extraction

This makes it more in line with the new Rx path.
It also makes the code more reusable because Rx
descriptor is more accessible.

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