net: fix circular dependency in of_mdio code

Commit 86f6cf41 (net: of_mdio: add of_mdiobus_link_phydev()) introduced a
circular dependency between libphy and of_mdio.

depmod: ERROR: <modroot>/kernel/drivers/net/phy/libphy.ko in
dependency cycle!
depmod: ERROR: <modroot>/kernel/drivers/of/of_mdio.ko in dependency cycle!

The problem is that of_mdio.c references &mdio_bus_type and libphy now
references of_mdiobus_link_phydev.

Fix this by not exporting of_mdiobus_link_phydev() from of_mdio.ko.
Make it a static function in mdio_bus.c instead.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Jeff Mahoney <jeffm@suse.com>
Fixes: 86f6cf41 (net: of_mdio: add of_mdiobus_link_phydev())
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>