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.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/phy.h

@@ -1900,10 +1900,8 @@ void phy_package_leave(struct phy_device *phydev);
 int devm_phy_package_join(struct device *dev, struct phy_device *phydev,
 			  int addr, size_t priv_size);
 
-#if IS_ENABLED(CONFIG_PHYLIB)
 int __init mdio_bus_init(void);
 void mdio_bus_exit(void);
-#endif
 
 int phy_ethtool_get_strings(struct phy_device *phydev, u8 *data);
 int phy_ethtool_get_sset_count(struct phy_device *phydev);