Move the netdevice notifier block register code in slave.c and provide
helpers for dsa.c to register and unregister it.

At the same time, check for errors since (un)register_netdevice_notifier
may fail.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>