net: hns3: do not initialize MDIO bus when PHY is inexistent

For some cases, PHY may not be connected to MDIO bus, then
the driver will initialize fail since MDIO bus initialization
fails.

This patch fixes it by skipping the MDIO bus initialization
when PHY is inexistent.

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>