net: atlantic: generate software timestamp just before the doorbell

Make sure the call of skb_tx_timestamp is as close as possible to the
doorbell.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Link: https://patch.msgid.link/20250510134812.48199-2-kerneljasonxing@gmail.com


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