net: stmmac: dwmac-sun8i: fix OF child-node lookup

Use the new of_get_compatible_child() helper to lookup the mdio-internal
child node instead of using of_find_compatible_node(), which searches
the entire tree from a given start node and thus can return an unrelated
(i.e. non-child) node.

This also addresses a potential use-after-free (e.g. after probe
deferral) as the tree-wide helper drops a reference to its first
argument (i.e. the mdio-mux node). Fortunately, this was inadvertently
balanced by a failure to drop the mdio-mux reference after lookup.

While at it, also fix the related mdio-internal- and phy-node reference
leaks.

Fixes: 634db83b ("net: stmmac: dwmac-sun8i: Handle integrated/external MDIOs")
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>