stmmac: dwmac-loongson: fix uninitialized variable in loongson_dwmac_probe()

The "mdio" variable is never set to false.  Also it should be a bool
type instead of int.

Fixes: 30bba69d

 ("stmmac: pci: Add dwmac support for Loongson")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>