wifi: mt76: connac: set correct muar_idx for mt799x chipsets

The MUAR (multicast unicast address) is an address mapping table that
participates in the process of searching WTBL entries. For mt799x
chipsets, the default muar index of BMC WTBL is 0xe.

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