tg3: allow TSO on vlan devices

Similar to commit 72dccb01 (bnx2: Update vlan_features)

In order to enable TSO on vlan devices, tg3 needs to update
dev->vlan_features.

Tested on HP NC326m (aka BCM5715S (rev a3))

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>