bluetooth: classic: shell: mitigate label followed by declaration

A recent change triggered a warning in LLVM that was promoted to error
when run with twister.

```shell
goep.c:102:3: error: label followed by a declaration is a C23 extension
  [-Werror,-Wc23-extensions]
  102 |                 int err;
```

Add a scope to the switch case so that the declaration has proper scope.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>