wifi: mt76: mt7915: fix beamforming availability check

Without this patch, when ap sets the tx stream number to 2,
ap won't send any beamforming packet.

Fixes: f89f297a ("mt76: mt7915: fix txbf starec TLV issues")
Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>