ath11k: calculate the correct NSS of peer for HE capabilities

When connected to 6G mode AP, it does not have VHT/HT capabilities,
so the NSS is not set, then it is 1 by default.

This patch is to calculate the NSS with supported HE-MCS and NSS set
of HE capabilities.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-01280-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1

Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211118095453.8030-1-quic_wgong@quicinc.com