cxgb4: Calculate len properly for LSO path

Commit 0034b298 ("cxgb4: Don't assume LSO only uses SGL path in t4_eth_xmit()")
introduced a regression where-in length was calculated wrongly for LSO path,
causing chip hangs.
So, correct the calculation of len.

Fixes: 0034b298 ("cxgb4: Don't assume LSO only uses SGL path in t4_eth_xmit()")
Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>