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>