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.

Co-authored-by: Naeem Bacha <naeem.bacha@audinate.com>
Signed-off-by: Chamira Perera <chamira.perera@audinate.com>