net: dsa: simplify netdevice events handling

Simplify the code handling the slave netdevice notifier call by
providing a dsa_slave_changeupper helper for NETDEV_CHANGEUPPER, and so
on (only this event is supported at the moment.)

Return NOTIFY_DONE when we did not care about an event, and NOTIFY_OK
when we were concerned but no error occurred, as the API suggests.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>