net: dsa: change scope of VLAN handlers

Change the scope of the switchdev VLAN object handlers from the DSA
slave device to the generic DSA port, so that the future port-wide API
can also be used for other port types, such as CPU and DSA links.

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>