mdio_bus: unhide mdio_bus_init prototype

mdio_bus: unhide mdio_bus_init prototype

mdio_bus_init() is either used as a local module_init() entry,
or it gets called in phy_device.c. In the former case, there
is no declaration, which causes a warning:

drivers/net/phy/mdio_bus.c:1371:12: error: no previous prototype for 'mdio_bus_init' [-Werror=missing-prototypes]

Remove the #ifdef around the declaration to avoid the warning..

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230516194625.549249-4-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>