net: dsa: qca8k: put MDIO controller OF node if unavailable

It was pointed out during the review [1] of commit e66bf63a ("net:
dsa: qca8k: skip MDIO bus creation if its OF node has status =
"disabled"") that we now leak a reference to the "mdio" OF node if it is
disabled.

This is only a concern when using dynamic OF as far as I can tell (like
probing on an overlay), since OF nodes are never freed in the regular
case. Additionally, I'm unaware of any actual device trees (in
production or elsewhere) which have status = "disabled" for the MDIO OF
node. So handling this as a simple enhancement.

[1] https://lore.kernel.org/netdev/CAJq09z4--Ug+3FAmp=EimQ8HTQYOWOuVon-PUMGB5a1N=RPv4g@mail.gmail.com/



Suggested-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>