ethtool.h: remind to update 802.3ad when adding new speeds

Each time a new speed is added, the bonding 802.3ad isn't updated. Add a
comment to remind the developer to update this driver.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>