mdio-thunder.c:undefined reference to `mdiobus_unregister'

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   32c1431eea4881a6b17bd7c639315010aeefa452
commit: 90eff9096c01ba90cdae504a6b95ee87fe2556a3 net: phy: Allow splitting MDIO bus/device support from PHYs
date:   3 months ago
config: x86_64-randconfig-s2-06120830 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        git checkout 90eff9096c01ba90cdae504a6b95ee87fe2556a3
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

   drivers/built-in.o: In function `thunder_mdiobus_pci_remove':
>> mdio-thunder.c:(.text+0x2a212f): undefined reference to `mdiobus_unregister'
>> mdio-thunder.c:(.text+0x2a2138): undefined reference to `mdiobus_free'
   drivers/built-in.o: In function `thunder_mdiobus_pci_probe':
   mdio-thunder.c:(.text+0x2a22e7): undefined reference to `devm_mdiobus_alloc_size'
   mdio-thunder.c:(.text+0x2a236f): undefined reference to `of_mdiobus_register'

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