batman-adv: Define module rtnl link name

The batman-adv module can automatically be loaded when operations over the
rtnl link are triggered. This requires only the correct rtnl link name in
the module header.

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