batman-adv: drop useless argument seqno in neighbor creation

the sequence number is not stored in struct neigh_node,
therefore there is no need to pass such value to the
neigh_node creation procedure.

At the moment the value is only used by a debug message, but
given the fact that the seqno is not related to the neighbor
object, it is better to print it elsewhere.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>