net: mdiobus: fwnode_mdiobus_register_phy() rework error handling

Rework error handling as preparation for PSE patch. This patch should
make it easier to extend this function.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>