net: dsa: use port's cpu_dp when creating a slave

When dsa_slave_create is called, the related port already has a CPU port
assigned to it, available in its cpu_dp member. Use it instead of the
unique tree cpu_dp.

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>