ixgb: fix cache miss due to miscalculation

Reduce writeback threshold by 1. We were instructing the hardware to
wait until the 17th descriptor which went over the cache line limit.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke.jan.h.kok@intel.com>
1 file changed