cxgb4: Don't assume LSO only uses SGL path in t4_eth_xmit()

Also, modify is_eth_imm() to return header length so it doesn't
have to be recomputed in calc_tx_flits().

Based on original work by Mike Werner <werner@chelsio.com>

Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>