net-timestamp: Update skb_complete_tx_timestamp comment

After "62bccb8c net-timestamp: Make the clone operation stand-alone from phy
timestamping" the hwtstamps parameter of skb_complete_tx_timestamp() may no
longer be NULL.
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Cc: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2884,11 +2884,11 @@ static inline bool skb_defer_rx_timestamp(struct sk_buff *skb)
  *
  * PHY drivers may accept clones of transmitted packets for
  * timestamping via their phy_driver.txtstamp method. These drivers
- * must call this function to return the skb back to the stack, with
- * or without a timestamp.
+ * must call this function to return the skb back to the stack with a
+ * timestamp.
  *
  * @skb: clone of the the original outgoing packet
- * @hwtstamps: hardware time stamps, may be NULL if not available
+ * @hwtstamps: hardware time stamps
  *
  */
 void skb_complete_tx_timestamp(struct sk_buff *skb,