net: ethernet: renesas: rswitch: Fix getting mac address from device tree

To get mac address from device tree which is from each ethernet-port,
fix the first argument of of_get_ethdev_address().

Fixes: 3590918b

 ("net: ethernet: renesas: Add support for "Ethernet Switch"")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>