net: dsa: assign a CPU port to DSA port

DSA ports also need to have a dedicated CPU port assigned to them,
because they need to know where to egress frames targeting the CPU,
e.g. To_Cpu frames received on a Marvell Tag port.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>