batman-adv: iv_ogm_can_aggregate, code readability

This patch tries to increase code readability by negating the first if
block and rearranging some of the other conditional blocks. This way we
save an indentation level, we also save some allocation that is not
necessary for one of the conditions.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>