stmmac: dwmac-loongson: Fix unsigned comparison to zero

plat->phy_interface is unsigned integer, so the condition
can't be less than zero and the warning will never printed.

Fixes: 30bba69d

 ("stmmac: pci: Add dwmac support for Loongson")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>