wifi: mt76: mt7996: fix beamform mcu cmd configuration

The bf_num field represents how many bands can support beamform, so set
the value to 3, and bf_bitmap represents the bitmap of bf_num.

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>