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>

drivers/net/ixgb/ixgb.h

@@ -111,7 +111,7 @@ struct ixgb_adapter;
 #define IXGB_RXBUFFER_16384 16384
 
 /* How many Rx Buffers do we bundle into one write to the hardware ? */
-#define IXGB_RX_BUFFER_WRITE	4	/* Must be power of 2 */
+#define IXGB_RX_BUFFER_WRITE	8	/* Must be power of 2 */
 
 /* only works for sizes that are powers of 2 */
 #define IXGB_ROUNDUP(i, size) ((i) = (((i) + (size) - 1) & ~((size) - 1)))