We can build modular code that uses mdiobus_register_board_info() which would
lead to linking failure since this symbol is not expoerted.
Fixes: 648ea0134069 ("net: phy: Allow pre-declaration of MDIO devices") Signed-off-by: Florian Fainelli <f.fainelli@gmail.com> Reviewed-by: Andrew Lunn <andrew@lunn.ch> Signed-off-by: David S. Miller <davem@davemloft.net>