When DNP3 heuristics are disabled, relax the conditions on
accepting a packet as DNP3.

Bug: 10287
Change-Id: I222ec885186447c8a72eaf11cebacff8b9b79fad
Reviewed-on: https://code.wireshark.org/review/3092
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>