Described what the port_vlan_filtering function is supposed to
accomplish.

Fixes: fb2dabad ("net: dsa: support VLAN filtering switchdev attr")
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Reviewed-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>