amd-xgbe: fix unused variable compilation warning in phylib driver

Fix following compilation warning:
[...]
  CC      drivers/net/phy/amd-xgbe-phy.o
drivers/net/phy/amd-xgbe-phy.c:1353:30: warning:
‘amd_xgbe_phy_ids’ defined but not used [-Wunused-variable]
 static struct mdio_device_id amd_xgbe_phy_ids[] = {
                              ^
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>