lan743x: correctly handle chips with internal PHY

lan743x: correctly handle chips with internal PHY

Commit e625dae96783 ("lan743x: Added fixed link and RGMII support")
assumes that chips with an internal PHY will never have a devicetree
entry. This is incorrect: even for these chips, a devicetree entry
can be useful e.g. to pass the mac address from bootloader to chip:

    &pcie {
            status = "okay";

            host@0 {
                    reg = <0 0 0 0 0>;

                    #address-cells = <3>;
                    #size-cells = <2>;

                    lan7430: ethernet@0 {
                            /* LAN7430 with internal PHY */
                            compatible = "microchip,lan743x";
                            status = "okay";
                            reg = <0 0 0 0 0>;
                            /* filled in by bootloader */
                            local-mac-address = [00 00 00 00 00 00];
                    };
            };
    };

If a devicetree entry is present, the driver will not attach the chip
to its internal phy, and the chip will be non-operational.

Fix by tweaking the phy connection algorithm:
- first try to connect to a phy specified in the devicetree
  (could be 'real' phy, or just a 'fixed-link')
- if that doesn't succeed, try to connect to an internal phy, even
  if the chip has a devnode

Tested on a LAN7430 with internal PHY. I cannot test a device using
fixed-link, as I do not have access to one.

Fixes: e625dae96783 ("lan743x: Added fixed link and RGMII support")
Tested-by: Sven Van Asbroeck <thesven73@gmail.com> # lan7430
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Link: https://lore.kernel.org/r/20201108171224.23829-1-TheSven73@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>