mt76: mt7915: fix the nss setting in bitrates

without this change, the fixed MCS only supports 1 Nss.

Fixes: 70fd1333 ("mt76: mt7915: rework .set_bitrate_mask() to support more options")
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>