wifi: mt76: connac: update nss calculation in txs

The hardware reports the number of transmitting antennas not the actual
number of spatial streams

Co-developed-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>