TRILL ISIS: SubTLVs of Router Capability TLV are valid for MT-Capability TLV as well

Change-Id: Idf466c021b1b188d9c6d55ba73a25257ba352412
Reviewed-on: https://code.wireshark.org/review/3803
Petri-Dish: Evan Huus <eapache@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>