net: dsa: use dedicated CPU port

Each port in DSA has its own dedicated CPU port currently available in
its parent switch's ds->ports[port].cpu_dp. Use it instead of getting
the unique tree CPU port, which will be deprecated soon.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>