LLDP: TIA Network Policy Unknown Policy Flag Decode is not correct

Wrong length for field...

Bug:10512
Change-Id: I416a34d5f50cc0e842612a084c95d885eea68ef1
Reviewed-on: https://code.wireshark.org/review/4377
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>