net: ethernet: sunplus: add missing of_node_put() in spl2sw_mdio_init()

of_get_child_by_name() returns device node pointer with refcount
incremented. The refcount should be decremented before returning
from spl2sw_mdio_init().

Fixes: fd3040b9

 ("net: ethernet: Add driver for Sunplus SP7021")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Wells Lu <wellslutw@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>