net: ibm: emac: remove mii_bus with devm

Switching to devm management of mii_bus allows to remove
mdiobus_unregister calls and thus avoids needing a mii_bus global struct
member.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20240912024903.6201-5-rosenp@gmail.com


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