net: dsa: qca8k: consistently use "ret" rather than "err" for error codes

It was pointed out during the review [1] of commit 68e1010c ("net:
dsa: qca8k: put MDIO bus OF node on qca8k_mdio_register() failure") that
the rest of the qca8k driver uses "int ret" rather than "int err".

Make everything consistent in that regard, not only
qca8k_mdio_register(), but also qca8k_setup_mdio_bus().

[1] https://lore.kernel.org/netdev/qyl2w3ownx5q7363kqxib52j5htar4y6pkn7gen27rj45xr4on@pvy5agi6o2te/



Suggested-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>