wifi: mt76: connac: introduce mt76_connac_spe_idx()

Add mt76_connac_spe_idx() for antenna mask to spatial extension
index conversion. This is used to support flexible control of
tx antenna.

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