ixgb: combine more rx descriptors to improve performance

We experimented with more descriptor buffer writebacks and found that
values larger than 8 give HW problems, but 8 is safe and gives us some
improved performance.

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