i40e: change Rx hang message into a WARN_ONCE

Use WARN_ONCE in order to highlight the issue, but don't display
a warning every time. The user should be able to see the ethtool counter
we created if necessary to see how often it is occurring.

Change-ID: I40c4ea159819b64a7d33b7f5716749089791533a
Signed-off-by: NJacob Keller <jacob.e.keller@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_ptp.c

@@ -289,9 +289,7 @@ void i40e_ptp_rx_hang(struct i40e_vsi *vsi)
 		rd32(hw, I40E_PRTTSYN_RXTIME_H(3));
 		pf->last_rx_ptp_check = jiffies;
 		pf->rx_hwtstamp_cleared++;
-		dev_warn(&vsi->back->pdev->dev,
-			 "%s: clearing Rx timestamp hang\n",
-			 __func__);
+		WARN_ONCE(1, "Detected Rx timestamp register hang\n");
 	}
 }