net: dsa: add slave notify helper

Both DSA slave create and destroy functions call call_dsa_notifiers with
respectively DSA_PORT_REGISTER and DSA_PORT_UNREGISTER and the same
dsa_notifier_register_info structure.

Wrap this in a dsa_slave_notify helper so prevent cluttering these
functions.

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>