igb: Prevent dropped Tx timestamps via work items and interrupts.

In rare circumstances, it's possible a descriptor writeback will occur
before a timestamped Tx packet will go out on the wire, leading to the
driver believing the hardware failed to timestamp the packet. Schedule a
work item for 82576 and use the available time sync interrupt registers
on 82580 and above to account for this.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>