mt76: mt7663: fix up BMC entry indicated to unicmd firmware

BMC entry for MT7663 unicmd firmware should be a broadcast/multicast entry,
not a unicast entry, that is GTK rekey offload would rely on.

Fixes: 13886067 ("mt76: mt7615: add more uni mcu commands")
Cc: Soul Huang <Soul.Huang@mediatek.com>
Suggested-by: YF Luo <Yf.Luo@mediatek.com>
Co-developed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>