net: dsa: use "err" instead of "ret" in dsa_port_probe

DM DSA uses "err" for error code values, so use this consistently.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Tested-by: Michael Walle <michael@walle.cc>