net: dsa: qca8k: unify bus id naming with legacy and OF mdio bus

Add support for multiple switch with OF mdio bus declaration.
Unify the bus id naming and use the same logic for both legacy and OF
mdio bus.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>