mt76: mt76x02u: enable multi-vif support

Enable multi-interface support for mt76x02u driver. For the moment
allow max two concurrent interfaces in order to preserve enough room
for ps traffic since we are using beacon slots for it.
I have successfully tested the following configuration:
- AP + STA
- AP0 + AP1

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>