ixgbe: fix X540 Completion timeout

On topologies including few levels of PCIe switching X540 can run into an
unexpected completion error.  We get around this by waiting after enabling
loopback a sufficient amount of time until Tx Data Fetch is sent.  We then
poll the pending transaction bit to ensure we received the completion.  Only
then do we go on to clear the buffers.

Signed-of-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>