ixgb: rx cleanup performance improvements

rx cleanup should look more like our other drivers that have evolved
to nicer performance levels over time.  Changes consist of refilling
tx buffers to hardware more often, some minor assignment cleanups.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>