net: dsa: remove useless args of dsa_slave_create

dsa_slave_create currently takes 4 arguments while it only needs the
related dsa_port and its name. Remove all other arguments.

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>