Remove use of pinfo->private_data (which doesn't appear to be used anyway) and pass it as dissector data.

Some other minor cleanup while in the neighborhood.

Change-Id: I1b0c0567488fa350c14d21c5f5e4cb9746177af1
Reviewed-on: https://code.wireshark.org/review/5447
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>