ath10k: Set proper nss value for the peer

It was found during testing the nss calculation does not
cover all corner cases. Station could request eq. only MCS8
and MCS9 (nss=2 specific). Next num_rates=2 so the driver
sets nss=(max((2+7)/8, 1))=1. Which is wrong. The in-driver
calculation was introduced prior (commit ddcc347b
mac80211: fix rx_nss calculation for drivers with hw rc).
Since it's fixed, use mac80211 provided value from now.
End user will experience lower throuhputs than expected
if the nss is wrongly calculated.

Signed-off-by: Marek Kwaczynski <marek.kwaczynski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>