ath10k: correct the array index from mcs index for HT mode for QCA6174

ath10k: correct the array index from mcs index for HT mode for QCA6174

The mcs index of HT mode is 0 to 31, please refer http://mcsindex.com/.
Its spatial stream(Nss) number is from 1 to 4, mcs index is 0~7 for
Nss=1, 8~15 for Nss=2, 16~23 for Nss=3 and 24~31 is for Nss=4.

The mcs is reported from firmware in wmi_tlv_peer_stats_info of
event WMI_TLV_PEER_STATS_INFO_EVENTID, its range is from 0~15 for
QCA6174 SDIO and PCIe. It is for both Nss=1 and Nss=2, and it has 2
rate table supported_ht_mcs_rate_nss1 and supported_ht_mcs_rate_nss2
in ath10k, they are for Nss=1 and Nss=2, each table has 8 rates.

It need to find the matched row number with the mcs index, for example,
mcs index is 2, it is <=7, so it is Nss=1, and match row 2 in table of
Nss=1. If mcs index is 12, it is >= 8 and <= 15, so it is Nss=2, it
match row 4(12-8) in table of Nss=2. If mcs index is >=16, it is for
Nss=3/4, it need to add rate table, so it is not support in current
ath10k.

This patch is to find the row number in rate table of Nss=1 or Nss=2
with the mcs index reported from firmware.

This patch only effect the chips which supports_peer_stats_info of its
hw_params is true, it is true only for QCA6174 currently.

Tested-on: QCA6174 hw3.2 SDIO WLAN.RMH.4.4.1-00048
Tested-on: QCA6174 hw3.2 PCI WLAN.RM.4.4.1-00110-QCARMSWP-1

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1597392294-13124-1-git-send-email-wgong@codeaurora.org