net: ipv6: Fix not initializing delayed work

net_ipv6_nbr_add shall init the delayed work as nbr_new does, so this
unifies both codes into nbr_init which does take care of initializing
the fields properly.

Change-Id: I91746276d346a3dc3c36be20d49bcf1968245fc5
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>