net: dsa: lantiq_gswip: fix OF child-node lookups

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: 14fceff4 ("net: dsa: Add Lantiq / Intel DSA driver for vrx200")
Cc: stable <stable@vger.kernel.org>	# 4.20
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>