igb: adjust PTP timestamps for Tx/Rx latency

Table 7-62 on page 338 of the i210 datasheet lists TX and RX latencies
for the various speeds the chip supports.  To give better PTP timestamp
accuracy, adjust the timestamps by the amounts Intel gives based on
current link speed.

Signed-off-by: Nathan Sullivan <nathan.sullivan@ni.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>