net: nixge: Fix Kconfig warning with OF_MDIO

Fix Kconfig warning with OF_MDIO where OF_MDIO was
selected unconditionally instead of only when
OF is actually enabled.

Fixes 7e8d5755 ("net: nixge: Add support for 64-bit platforms")
Suggested-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NMoritz Fischer <mdf@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/ni/Kconfig

@@ -20,7 +20,7 @@ config NI_XGE_MANAGEMENT_ENET
 	tristate "National Instruments XGE management enet support"
 	depends on HAS_IOMEM && HAS_DMA
 	select PHYLIB
-	select OF_MDIO
+	select OF_MDIO if OF
 	help
 	  Simple LAN device for debug or management purposes. Can
 	  support either 10G or 1G PHYs via SFP+ ports.