net: 8139too: Initial necessary vlan_features to support vlan

Offload setting of vlan device requires
vlan_features to be initialized.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>