cxgb4: Update comment for min_mtu

We have lost a comment for minimum mtu value set for netdevice with
'commit d894be57 ("ethernet: use net core MTU range checking in
more drivers"). Updating it accordingly.
Signed-off-by: NArjun Vynipadath <arjun@chelsio.com>
Signed-off-by: NGanesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c

@@ -5024,7 +5024,7 @@ static int init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
 		netdev->priv_flags |= IFF_UNICAST_FLT;
 
 		/* MTU range: 81 - 9600 */
-		netdev->min_mtu = 81;
+		netdev->min_mtu = 81;              /* accommodate SACK */
 		netdev->max_mtu = MAX_MTU;
 
 		netdev->netdev_ops = &cxgb4_netdev_ops;