Use the syscon lookup-by-phandle helper so that the reference taken by
of_parse_phandle() is released when done with the node.

Fixes: 5ed74140 ("net: stmmac: Add OXNAS Glue Driver")
Signed-off-by: NJohan Hovold <johan@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>