net: dsa: get port name at parse time

Get the optional "label" property and assign a default one directly at
parse time instead of doing it when creating the slave.

For legacy, simply assign the port name stored in cd->port_names.

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>