net: dsa: Don't add vlans when vlan filtering is disabled

The software bridge can be build with vlan filtering support
included. However, by default it is turned off. In its turned off
state, it still passes VLANs via switchev, even though they are not to
be used. Don't pass these VLANs to the hardware. Only do so when vlan
filtering is enabled.

This fixes at least one corner case. There are still issues in other
corners, such as when vlan_filtering is later enabled.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>