stmmac: dwmac-loongson: Make sure MDIO is initialized before use

Generic code will use mdio. If it is not initialized before use,
the kernel will Oops.

Fixes: 30bba69d ("stmmac: pci: Add dwmac support for Loongson")
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>