Staging: batman-adv: fix function prototype

In today linux-next I got a compile warning in staging/batman-adv.

This is due a struct bin_attribute read function prototype change and the driver was not updated.

This patch solves the issue

Signed-off-by: Javier Martinez Canillas <martinez.javier@gmail.com>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>