btavdtp: Add missing break;

Issue found by Clang Analyzer:
Value stored to 'codec_dissector' is never read

Change-Id: I7a9fc5e3ffd04c77cbbb8d5e5b885d8a29468cfd
Reviewed-on: https://code.wireshark.org/review/5330
Reviewed-by: Balint Reczey <balint@balintreczey.hu>