net: dsa: bcm_sf2: Add missing OF_MDIO dependency

bcm_sf2 does require the MDIO_BCM_UNIMAC driver which is now dependent
on OF_MDIO but also internally uses of_mdio.c provided routines which
are guarted with OF_MDIO.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 90eff90 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/Kconfig

@@ -11,7 +11,7 @@ config NET_DSA_MV88E6060
 
 config NET_DSA_BCM_SF2
 	tristate "Broadcom Starfighter 2 Ethernet switch support"
-	depends on HAS_IOMEM && NET_DSA
+	depends on HAS_IOMEM && NET_DSA && OF_MDIO
 	select NET_DSA_TAG_BRCM
 	select FIXED_PHY
 	select BCM7XXX_PHY