r8169: fix a typo in a comment

Replace "additonal" by "additional" in a comment.
Typo found by checkpatch.pl.
Signed-off-by: Corentin Musard <corentinmusard@gmail.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169_main.c

@@ -6321,7 +6321,7 @@ rtl8169_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats)
 	stats->multicast	= dev->stats.multicast;
 
 	/*
-	 * Fetch additonal counter values missing in stats collected by driver
+	 * Fetch additional counter values missing in stats collected by driver
 	 * from tally counters.
 	 */
 	if (pm_runtime_active(&pdev->dev))