wifi: mt76: mt7996: enable BSS_CHANGED_MCAST_RATE support

Similar to BSS_CHANGED_BASIC_RATES, this enables mcast rate
configuration through fixed rate tables.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Change-Id: Ifc305e8c7de9a7df4ad5f856e2097d721a886aaa
Signed-off-by: Felix Fietkau <nbd@nbd.name>