igb: Add timeout for PTP Tx work item

When transmitting a packet that must return a Tx timestamp, a work item
gets scheduled to poll for the Tx timestamp being completed in hardware.
Add a timeout on this work item of 15 seconds from when the driver gets the
skb, after which it will stop polling. Report via stats and system log if
this occurs.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Acked-by: Jacob Keller <Jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>