batman-adv: Remove ethtool msglevel functions

batadv devices don't support msglevel. The ethtool stubs therefore returned
that it isn't supported. But instead, the complete function can be dropped
to avoid that bogus values are shown in ethtool.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>