net/mlx4_en: Support software timestamping

Kernel software timestamping requires that the driver calls skb_tx_timestamp
just before passing the skb to the HW MAC layer. This patch adds this call.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>