net: lib: mdns_responder: Fix interface count check

The original idea was to check that we have enough network
interfaces in the system. The check needs to verify max IPv4
and IPv6 supported interfaces instead of always checking
IPv6 one.

Fixes: #66843
Coverity-CID: 334899

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>