wcn36xx: Add ieee80211 rx status rate information

Packet encoding, bandwidth and bitrate can be derived from the
wcn36xx rate_idx, part of the buffer descriptor.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1591961254-10243-1-git-send-email-loic.poulain@linaro.org