Skip parameter check for help command to avoid invalid error

If I run:

```
$ wp post list --post_type=post --help
Error: Parameter errors:
unknown --post_type parameter
```

Instead of an error the command above should load the help page for `wp post list` since --post_type is actually a valid parameter.

This error is happening because Subcommand::validate_args() on line 253 checks for valid parameters for `wp help` command instead of valid parameters for `wp post list` subcommand. Since I don't the internals of WP-CLI very well the only solution I can think of is add an if before the foreach to skip the check when in help command context.

Please let me know if there is a better way of fixing this issue and I will update the PR.