sfc: Disable VLAN filtering by default if not strictly required

If should be done after net_dev->hw_features initialization, to keep the
feature there to be able to enable it later using ethtool.

VLAN filtering is enforced and fixed if vPort requires usage of VLAN
filters to receive tagged traffic.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>