packet-hislip.c: various minor fixes

- Create/use an extended value-string;
- Clean up code relating to setting COL_PROTO & COL_INFO;
- Use consistent whitespace formatting.

Change-Id: I3087d959db09cd90c1a5dd53fe7af7baf999b76c
Reviewed-on: https://code.wireshark.org/review/3542
Reviewed-by: Bill Meier <wmeier@newsguy.com>