igb: Change Tx cleanup loop to do/while instead of for

This change makes it so that Tx cleanup is done in a do/while loop instead
of a for loop.  The main motivation behind this is the fact that we should
never be invoked with a budget less than 1 so we can skip checking the
budget before processing the first descriptor.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>