ionic: check for valid tx_mode on SKBTX_HW_TSTAMP xmit

Make sure the device is in a Tx offload mode before calling the
hwstamp offload xmit.
Signed-off-by: NShannon Nelson <snelson@pensando.io>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/pensando/ionic/ionic_txrx.c

@@ -1233,7 +1233,7 @@ netdev_tx_t ionic_start_xmit(struct sk_buff *skb, struct net_device *netdev)
 	}
 
 	if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP))
-		if (lif->hwstamp_txq)
+		if (lif->hwstamp_txq && lif->phc->ts_config_tx_mode)
 			return ionic_start_hwstamp_xmit(skb, netdev);
 
 	if (unlikely(queue_index >= lif->nxqs))