packet-hislip.c: Fix various bugs and deficiencies

- Remove an incorrect call to conversation_new()
  (which messed up TCP sequence number analysis).
- hislip Request/Response analysis must be done during 'pass1'
  (i.e. when 'flags.visited == 0');
  Not doing so caused various problems including
   tshark '1 pass' poor dissection.
- Remove 'if (tree)' around a call to expert_...().
- Don't show "unknown" for Async/Sync when the value is actually known.
- Simplify some code.
- Mark a field as GENERATED.

Change-Id: I286c12f52e5f73377bed3a2792f3ff0003e2785f
Reviewed-on: https://code.wireshark.org/review/3541
Petri-Dish: Bill Meier <wmeier@newsguy.com>
Reviewed-by: Bill Meier <wmeier@newsguy.com>