ixgbe: Add ptp work item to poll for the Tx timestamp

This patch copies the igb implementation of Tx timestamps, which uses a work
item to poll for the Tx timestamp. In addition it adds a timeout value of 15
seconds, after which it will stop polling.

This is necessary due to an issue with the descriptor being marked done before
the Tx timestamp event has occurred. These two events don't correlate, so using
the done bit on the descriptor as indication that the timestamp must already
have been taken leads to potentially dropped Tx timestamps (especially under
heavy packet load)

Reported-by: Matthew Vick <matthew.vick@intel.com>
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>