net: dsa: qca8k: put MDIO bus OF node on qca8k_mdio_register() failure

[ Upstream commit 68e1010c ]

of_get_child_by_name() gives us an OF node with an elevated refcount,
which should be dropped when we're done with it. This is so that,
if (of_node_check_flag(node, OF_DYNAMIC)) is true, the node's memory can
eventually be freed.

There are 2 distinct paths to be considered in qca8k_mdio_register():

- devm_of_mdiobus_register() succeeds: since commit 3b73a7b8 ("net:
  mdio_bus: add refcounting for fwnodes to mdiobus"), the MDIO core
  treats this well.

- devm_of_mdiobus_register() or anything up to that point fails: it is
  the duty of the qca8k driver to release the OF node.

This change addresses the second case by making sure that the OF node
reference is not leaked.

The "mdio" node may be NULL, but of_node_put(NULL) is safe.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>