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: NFrancois Romieu <romieu@fr.zoreil.com>
Acked-by: NTom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/phy/amd-xgbe-phy.c

@@ -1350,7 +1350,7 @@ static void __exit amd_xgbe_phy_exit(void)
 module_init(amd_xgbe_phy_init);
 module_exit(amd_xgbe_phy_exit);
 
-static struct mdio_device_id amd_xgbe_phy_ids[] = {
+static struct mdio_device_id __maybe_unused amd_xgbe_phy_ids[] = {
 	{ XGBE_PHY_ID, XGBE_PHY_MASK },
 	{ }
 };