net: dsa: don't stop at NOTIFY_OK when calling ds->ops->port_prechangeupper

dsa_slave_prechangeupper_sanity_check() is supposed to enforce some
adjacency restrictions, and calls ds->ops->port_prechangeupper if the
driver implements it.

We convert the error code from the port_prechangeupper() call to a
notifier code, and 0 is converted to NOTIFY_OK, but the caller of
dsa_slave_prechangeupper_sanity_check() stops at any notifier code
different from NOTIFY_DONE.

Avoid this by converting back the notifier code to an error code, so
that both NOTIFY_OK and NOTIFY_DONE will be seen as 0. This allows more
parallel sanity check functions to be added.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>