ixgb: fix bug in descriptor ring due to prefetch corruption

there was one more bug hidden in the prefetch routines in ixgb hardware
that force us to remove it completely.  Writebacks were being done on
descriptors with stale data due to internal hardware fifo corruption.

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>