net: dsa: qca8k: consolidate calls to a single devm_of_mdiobus_register()

__of_mdiobus_register() already calls __mdiobus_register() if the
OF node provided as argument is NULL. We can take advantage of that
and simplify the 2 code path, calling devm_of_mdiobus_register() only
once for both cases.

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>
Reviewed-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>