e1000e: fix numeric overflow in phc settime method

The PTP Hardware Clock settime function in the e1000e driver
computes nanoseconds from a struct timespec. The code converts the
seconds field .tv_sec by multiplying it with NSEC_PER_SEC. However,
both operands are of type long, resulting in an unintended overflow.
The patch fixes the issue by using the helper function from time.h.

CC: stable <stable@vger.kernel.org>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>