Staging: batman-adv: remove redundant struct declaration

The hardif_attr and the bat_attr struct share the same attributes,
hence it is not necessary to specify 2 different structs.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>