ptp: ixp46x: use helpers for converting ns to timespec

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

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>