ptp: ixgbe: use helpers for converting ns to timespec.

This patch changes the driver to use ns_to_timespec64() and
timespec64_to_ns() instead of open coding the same logic.

Compile tested only.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>