If the RX packet is timestamped by the hardware, the RX completion
record will contain the lower 32-bit of the timestamp.  This needs
to be combined with the upper 16-bit of the periodic timestamp that
we get from the timer.  The previous snapshot in ptp->old_timer is
used to make sure that the snapshot is not ahead of the RX timestamp
and we adjust for wrap-around if needed.

v2: Make ptp->old_time read access safe on 32-bit CPUs.
Reviewed-by: NEdwin Peer <edwin.peer@broadcom.com>
Signed-off-by: NPavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: NMichael Chan <michael.chan@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>