igb: Add update to last_rx_timestamp in Rx rings

In order to support a more accurate check for a PTP Rx hang where the
device can no longer timestamp received packets, we need to update, per
ring, when the last Rx timestamp was. Because of how the PTP Rx hang logic
works, the current logic is valid, but properly updating the ring variable
increases the accuracy of the check.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>