i40e: enable descriptor prefetch for VFs

As recommended by the hardware guys, enable descriptor prefetch for
rings belonging to VFs. This matches a change already made for ring
belonging to the PF.

Change-ID: Idcc4dc7064bc9144ea81f5701ad07b7ecd11ba49
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_virtchnl_pf.c

@@ -354,6 +354,7 @@ static int i40e_config_vsi_rx_queue(struct i40e_vf *vf, u16 vsi_idx,
 	rx_ctx.tphhead_ena = 1;
 	rx_ctx.lrxqthresh = 2;
 	rx_ctx.crcstrip = 1;
+	rx_ctx.prefena = 1;
 
 	/* clear the context in the HMC */
 	ret = i40e_clear_lan_rx_queue_context(hw, pf_queue_id);