ath10k: fix reported HT MCS rates with NSS > 1

ath10k: fix reported HT MCS rates with NSS > 1

The QCA4019 firmware 10.4-3.2.1-00050 reports only HT MCS rates between
0-9. But 802.11n MCS rates can be larger than that. For example a 2x2
device can send with up to MCS 15.

The firmware encodes the higher MCS rates using the NSS field. The actual
calculation is not documented by QCA but it seems like the NSS field can be
mapped for HT rates to following MCS offsets:

 * NSS 1: 0
 * NSS 2: 8
 * NSS 3: 16
 * NSS 4: 24

This offset therefore has to be added for HT rates before they are stored
in the rate_info struct.

Fixes: cec17c382140 ("ath10k: add per peer htt tx stats support for 10.4")
Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>