Revert "net: mdio: get/put device node during (un)registration"

This reverts commit cff9c565

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Revert based on feedback from Russell.

Link: https://lore.kernel.org/all/ZZPtUIRerqTI2%2Fyh@shell.armlinux.org.uk/
Signed-off-by: Jakub Kicinski <kuba@kernel.org>