batman-adv: aggregation checks should use the primary_if pointer

The packet aggregation needs to ensure that only compatible packets
are aggregated. Some of the checks are based on the interface number
while assuming that the first interface also is the primary interface
which is not always the case.
This patch addresses the issue by using the primary_if pointer.

Reported-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>