net: wifi: shell: enhance consistency in code style

Enhancing code consistency provides cognitive leverage with
the following changes:
- Unified the order of declaration for `opt`, `opt_index`,
  `state`, and `long_options`.
- Unified the wrapping of `getopt_long` calls, regardless of
  the length of the `options` string.
- Renamed `option_index` to `opt_index` for consistency.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>