drivers: net: xgene: Add backward compatibility

This patch adds xgene_enet_check_phy_hanlde() function that checks whether
MDIO driver is probed successfully and sets pdata->mdio_driver to true.
If MDIO driver is not probed, ethernet driver falls back to backward
compatibility mode.

Since enum xgene_enet_cmd is used by MDIO driver, removing this from
ethernet driver.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>