i40evf: add comment

Add a comment to reduce confusion.

Change-ID: I3d5819c0f3f5174680442ae54398a073d4a61f4f
Signed-off-by: NMitch Williams <mitch.a.williams@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40evf/i40evf_main.c

@@ -2153,6 +2153,11 @@ static int i40evf_close(struct net_device *netdev)
 	adapter->state = __I40EVF_DOWN_PENDING;
 	i40evf_free_traffic_irqs(adapter);
 
+	/* We explicitly don't free resources here because the hardware is
+	 * still active and can DMA into memory. Resources are cleared in
+	 * i40evf_virtchnl_completion() after we get confirmation from the PF
+	 * driver that the rings have been stopped.
+	 */
 	return 0;
 }