mt76: mt7615: add debugfs knob for setting extended local mac addresses

This is primarily for testing and can be used in combination with monitor
mode to make the card respond to packets sent to a specific MAC address.
For now this is only exposed as a debug/testing feature, later on the
approach might be used to support more concurrent station interfaces

Signed-off-by: Felix Fietkau <nbd@nbd.name>