[][MAC80211][core][mt76: connac: update nss calculation in txs]

[Description]
Fix the nss calculation. The nss is nsts divided by 2 when stbc is 1.

[Release-log]
N/A

Change-Id: I243c7873b488297ff511e63f1f0102dd471637ff
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6655696
Build: srv_hbgsm110
1 file changed