net: dsa: qca8k: use "dev" consistently within qca8k_mdio_register()

Accessed either through priv->dev or ds->dev, it is the same device
structure. Keep a single variable which holds a reference to it, and use
it consistently.

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>