i40e: fix race in hang check

The driver was having some issues with false Tx hang detection. This
makes the driver a little more direct with the checks for progress
forward by directly checking the head write back address and tail register
when determining progress.  This avoids Tx hangs where the software
gets behind, because we are directly checking hardware state when
determining hang state.

Change-ID: I774f0e861c9e8ab5ccb213634100fe15440ae24a
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>