ixgbe: change PTP NSECS_PER_SEC to IXGBE_PTP_PPS_HALF_SECOND

The PPS signal is not correct, as it generates a one half HZ clock
signal, as it only generates one level change per second. To generate a
full clock, we need two level changes per second. Also, change the name
of the #define, in order to prevent confusion between it and
NSEC_PER_SEC which is not guaranteed to be a 64bit value.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>