net: dhcpv4: fix deadlock issue

There is a deadlock issue when calling stop using address conflict
detection. This is due to the fact that some net_mgmt events are fired
and trigger the dhcpv4_acd_event_handler() with lock held even if they
are of no interest for this callback.

Therefore, before acquiring the lock, make sure the event we received
is one we are expecting.

Also, do the same for dhcpv4_iface_event_handler().

Signed-off-by: Mathieu Anquetin <mathieu.anquetin@groupe-cahors.com>