net: dsa: change scope of VLAN filtering setter

Change the scope of the switchdev VLAN filtering attribute setter 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>