i40e/i40evf: enable prefetch of Tx descriptors during cleanup

Performance can be improved a bit by imitating ixgbe and using
prefetch to get us the next Tx descriptor.

Change-ID: Ice7ffd4cd0ce87c35295059bdb7972a7f53723aa
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
index a60b840..f5a50b9 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
@@ -770,6 +770,8 @@
 			tx_desc = I40E_TX_DESC(tx_ring, 0);
 		}
 
+		prefetch(tx_desc);
+
 		/* update budget accounting */
 		budget--;
 	} while (likely(budget));