openflow: Minor changes

- Remove unneeded/undefined forward declarations;
- Create/use some extended value-strings;
- Do some whitespace changes.

Change-Id: I7961864f0e404e141d2887584751116011f8d941
Reviewed-on: https://code.wireshark.org/review/5490
Reviewed-by: Bill Meier <wmeier@newsguy.com>