stmmac: dwmac-loongson: fix invalid mdio_node

stmmac: dwmac-loongson: fix invalid mdio_node

In current code "plat->mdio_node" is always NULL, the mdio
support is lost as there is no "mdio_bus_data". The original
driver could work as the "mdio" variable is never set to
false, which is described in commit <b0e03950dd71> ("stmmac:
dwmac-loongson: fix uninitialized variable ......"). And
after this commit merged, the "mdio" variable is always
false, causing the mdio supoort logic lost.

Fixes: 30bba69d7db4 ("stmmac: pci: Add dwmac support for Loongson")
Signed-off-by: Liu Peibao <liupeibao@loongson.cn>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20221101060218.16453-1-liupeibao@loongson.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>