wifi: mt76: connac: fix txd multicast rate setting

The vif->bss_conf.mcast_rate should be applied to multicast data frame
only.

Fixes: 182071cd

 ("mt76: connac: move connac2_mac_write_txwi in mt76_connac module")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>