Improve filterability of RSVP dissector.

Eliminated many, but not all proto_tree_add_text calls.

Change-Id: Ic904a0019a9d5d64d47b562bd282f1e6fad617cb
Reviewed-on: https://code.wireshark.org/review/4366
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>