batman-adv: a delayed_work has to be initialised once

A delayed_work struct does not need to be initialized each
every time before being enqueued. Therefore the
INIT_DELAYED_WORK() macro should be used during the
initialization process only.

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