net: mlx4: add SOF_TIMESTAMPING_TX_SOFTWARE flag when getting ts info

[ Upstream commit b86bcfee30576b752302c55693fff97242b35dfd ]

As mlx4 has implemented skb_tx_timestamp() in mlx4_en_xmit(), the
SOFTWARE flag is surely needed when users are trying to get timestamp
information.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20250510093442.79711-1-kerneljasonxing@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>