net: ipv6: set NET_ADDR_PREFERRED before network event if !DAD

Set NET_ADDR_PREFERRED before network event is generated, so that conn_mgr
properly generates NET_EVENT_L4_CONNECTED and NET_EVENT_L4_IPV6_CONNECTED
events.

Fixes: 1a5e13a7 ("net: if: Release the interface lock early when
  starting IPv4 ACD")
Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>
(cherry picked from commit 7290dd2a)