net: phy: micrel: Find Micrel PHY node correctly

In some of the device trees that specify skew values for KSZ90x1 PHYs
the values are stored (incorrectly) in the MAC node, whereas in others
it is in an 'ethernet-phy' subnode. Previously the code would fail to
find and program these skew values, so this commit changes it to look
for an "ethernet-phy" subnode first, and revert to looking in the MAC
node if there isn't one.

The device trees affected (where the skew values are in a subnode) are
imx6qdl-icore-rqs.dtsi, r8a77970-eagle.dts, r8a77990-ebisu.dts,
r8a77995-draak.dts, salvator-common.dtsi, sama5d3xcm.dtsi,
sama5d3xcm_cmp.dtsi, socfpga_cyclone5_vining_fpga.dts,
socfpga_stratix10_socdk.dts and ulcb.dtsi. Before this change the skew
values in these device trees would be ignored.

The device trees where the skew values are in the MAC node are
socfpga_arria10_socdk.dtsi, socfpga_arria5_socdk.dts,
socfpga_cyclone5_de0_nano_soc.dts, socfpga_cyclone5_de10_nano.dts,
socfpga_cyclone5_de1_soc.dts, socfpga_cyclone5_is1.dts,
socfpga_cyclone5_socdk.dts, socfpga_cyclone5_sockit.dts. These should be
unaffected by this change.

The changes were tested on a sama5d3xcm.

Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>