ath10k: fix NSS reporting in RX

NSTS values reported in the VHT-SIG-A1 are 0
through 7 but they actually describe number of
streams 1 through 8.

1SS frames were dropped. This patch fixes this.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>