bnx2x: add transmit timestamping support

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Eilon Greenstein <eilong@broadcom.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>