teaming: fix vlan_features computing

__team_compute_features() uses netdev_increment_features() to
combine vlan_features of slaves into vlan_features of the team.
As netdev_increment_features() only adds most features and we
start with TEAM_VLAN_FEATURES, we can end up with features none
of the slaves provided.

Initialize vlan_features only with the flags which are both in
TEAM_VLAN_FEATURES and NETIF_F_ALL_FOR_ALL. Right now there is
no such feature so that we actually initialize vlan_features
with zero but stating it explicitely will make the code more
future proof.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>