ath10k: add support for qca6174 Rx descriptors

The QCA6174 chip has an extra 4 bytes in
rx_ppdu_end structure which is used in htt_rx_desc
and HTT Rx ring offset setup. This is necessary
for correct Rx for QCA6174 (otherwise Rx
descriptors are overwritten and corrupted).

This means QCA988X will have an extra 4 byte
padding in Rx descriptor layout which is harmless.

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