net: l2: ethernet: arp: Simplify ACD case

In case of ACD Probe/Announcement, all we need is to generate ARP
packet, we don't really want any cache entries to be created or searched
for. There was a bug, that a cache entry was created for the
Announcement sent, resulting in skipped ARP packet generation and
malformed packet being sent by the ACD module.

Therefore, simplify all this, by simply returning early in case of
conflict detection packets.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>