Fix Kconfig warning with OF_MDIO where OF_MDIO was
selected unconditionally instead of only when
OF is actually enabled.
Fixes 
7e8d5755be0e ("net: nixge: Add support for 64-bit platforms")
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
        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.