net: rpl: Make sure k_delayed_work_init is only called once

k_delayed_work_init shall only be called once per work and not every
time before as that can trash the workqueue list if the work is pending.

Change-Id: Ib04b6689c59d03328687b0b579e80012dbd2f853
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>