Fixed setting 'compound' option of sonata_type_model

This PR fixes bugs with `sonata_type_model` that occurs with options 
- `multiple=true` + `expanded=false` - instead of multiple select no data were displayed,
- `multiple=false` + `expanded=true` - instead of radio buttons exception `You cannot add children to a simple form...` occurs.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT