In the previous code, TX timestamp accuracy was bad because the irq
handler got the timestamp from the timestamp register at that time.

This hardware has "Timestamp capture" feature which can store
each TX timestamp into the timestamp descriptors. To improve
TX timestamp accuracy, implement timestamp descriptors' handling.
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>