bnx2: Always set ETH_FLAG_TXVLAN

TSO does not work if the VLAN tag is in the packet (non-accelerated).
We may be able to remove this restriction in future firmware.

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