ixgbe: Use ring values to test for Tx pending

This patch simplifies the check for Tx pending traffic and makes it more
holistic as there being any difference between next_to_use and
next_to_clean is much more informative than if head and tail are equal, as
it is possible for us to either not update tail, or not be notified of
completed work in which case next_to_clean would not be equal to head.

In addition the simplification makes it so that we don't have to read
hardware which allows us to drop a number of variables that were previously
being used in the call.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>