TRILL TREE Sub-TLV fixes

1) Fix starting tree number
2) Display Nicknames both in hex and dec

Change-Id: If58d034e98429019d769ebe7be635a296e8ef18d
Reviewed-on: https://code.wireshark.org/review/3687
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>