net: phy: prevent stale pointer dereference in phy_init()

mdio_bus_init() and phy_driver_register() both have error paths, and if
those are ever hit, ethtool will have a stale pointer to the
phy_ethtool_phy_ops stub structure, which references memory from a
module that failed to load (phylib).

It is probably hard to force an error in this code path even manually,
but the error teardown path of phy_init() should be the same as
phy_exit(), which is now simply not the case.

Fixes: 55d8f053

 ("net: phy: Register ethtool PHY operations")
Link: https://lore.kernel.org/netdev/ZLaiJ4G6TaJYGJyU@shell.armlinux.org.uk/
Suggested-by: Russell King (Oracle) <linux@armlinux.org.uk>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230720000231.1939689-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>