ptp_pch: use helpers function for converting between ns and timespec

use ns_to_timespec64() and timespec64_to_ns() instead of open coding

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