wifi: mt76: connac: add beacon duplicate TX mode support for mt7996

For connac3 chipsets, setting of spe_idx is moved from TX descriptor to
the fixed rate table. This patch implements the setting to support
duplicate TX mode for beacon.

Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>