ixgbe: Update ptp_overflow check comment and jiffies

This patch fixes the comment on ptp_overflow_check to match up with what is
currently used as the parameters. Also change the jiffies check to use
time_is_after_jiffies macro.

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>