net: phy: Fix MDIO_THUNDER dependencies

After commit 90eff909 ("net: phy: Allow splitting MDIO
bus/device support from PHYs") we could create a configuration where
MDIO_DEVICE=y and PHYLIB=m which leads to the following undefined
references:

 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'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 90eff909 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>