net: dsa: realtek-smi: fix OF child-node lookup

Use the new of_get_compatible_child() helper to look up child nodes to
avoid ever matching non-child nodes elsewhere in the tree.

Also fix up the related struct device_node leaks.

Fixes: d8652956 ("net: dsa: realtek-smi: Add Realtek SMI driver")
Cc: stable <stable@vger.kernel.org>     # 4.19: 36156f92
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>