net: dsa: change scope of notifier call chain

Change the scope of the fabric notification helper from the DSA slave to
the DSA port, since this is a DSA layer specific notion, that can be
used by non-slave ports (CPU and DSA).

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>