stm32cube: stm32h7: use transmit timestamp status flag.

Read timestamp only when status flag is set in Tx descriptor.
Fire transmit timestamp callbacks only when there is a valid
non-zero timestamp.

(cherry picked from commit d466dc8421ee0c6592bb5682aa93a671bc948107)

Original-Co-authored-by: Naeem Bacha <naeem.bacha@audinate.com>
Original-Signed-off-by: Chamira Perera <chamira.perera@audinate.com>
GitOrigin-RevId: d466dc8421ee0c6592bb5682aa93a671bc948107
Change-Id: Ic216bb5d72be4570721d79b0d550fc4d790cb194
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/hal_stm32/+/4720447
Commit-Queue: Yuval Peress <peress@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Yuval Peress <peress@google.com>
Tested-by: Yuval Peress <peress@google.com>