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