igb: Update RXDCTL/TXDCTL configurations

This change cleans up the RXDCTL and TXDCTL configurations and optimizes RX
performance by allowing back write-backs on all hardware other than 82576.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by:  Aaron Brown  <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/drivers/net/ethernet/intel/igb/igb.h b/drivers/net/ethernet/intel/igb/igb.h
index 265e151..577fd3e 100644
--- a/drivers/net/ethernet/intel/igb/igb.h
+++ b/drivers/net/ethernet/intel/igb/igb.h
@@ -100,11 +100,12 @@
  */
 #define IGB_RX_PTHRESH                     8
 #define IGB_RX_HTHRESH                     8
-#define IGB_RX_WTHRESH                     1
 #define IGB_TX_PTHRESH                     8
 #define IGB_TX_HTHRESH                     1
+#define IGB_RX_WTHRESH                     ((hw->mac.type == e1000_82576 && \
+					     adapter->msix_entries) ? 1 : 4)
 #define IGB_TX_WTHRESH                     ((hw->mac.type == e1000_82576 && \
-                                             adapter->msix_entries) ? 1 : 16)
+					     adapter->msix_entries) ? 1 : 16)
 
 /* this is the size past which hardware will drop packets when setting LPE=0 */
 #define MAXIMUM_ETHERNET_VLAN_SIZE 1522