packet-lg8979.c: Various minor changes

- tvb_length() --> tvb_reported_length();
- Add an XXX comment re use of 'get_length' for tcp_dissect_pdus();
- Remove a few unneeded initializers;
- Simplify/adjust code in a few places;
- Remove some boilerplate comments;
- Reformat some whitespace and long-lines.

Change-Id: I17b9750cc1bdb5140edc28efbae5bf0f6ec23b6e
Reviewed-on: https://code.wireshark.org/review/3538
Petri-Dish: Bill Meier <wmeier@newsguy.com>
Reviewed-by: Bill Meier <wmeier@newsguy.com>