mt76: unify add_interface

Make common add_interface for mt76x0 and mt76x2e. This change behavior
for mt76x0, but it should work with the new implementation.

mt76x2u has different implementation. Maybe it can use common one,
but for now leave it as is.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>