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>