net: dsa: loop: Configure VLANs while not filtering

Since this is a mock-up driver with no real data path for now, but we
will have one at some point, enable VLANs while not filtering.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>