ath10k: fix station nss computation

If station advertises diffferent NSS capabilities in Rx_mcs set
of HT and VHT IEs in assoc req, the current NSS computation
logic configures the NSS support only based on Rx_mcs set of
HT capabilities in the driver. This is configuring the station
NSS capabilities incorreclty in the target.

For example, if station advertise Rx_mcs set as 2 spatial streams
in HT capabilities and 1 spatial streams in VHT capabilities in
assoc request, as per current logic we are calculating nss from
HT capabilities and the driver sets peer_num_spatial_streams as
2 for the station which is configured in VHT 1*1.

This patchs fix this issue by calculating the nss from VHT cap if
station supports vht.

Signed-off-by: Venkateswara Rao Naralasetty <c_vnaral@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>