Return the child schema for the correct const version.

def get_schema(self, protocol_version):
        """Return the child schema for the correct const version."""
        const = get_const(protocol_version)
        custom_schema = vol.Schema({
            typ.value: const.VALID_SETREQ[typ]
            for typ in const.VALID_TYPES[const.Presentation.S_CUSTOM]})
        return custom_schema.extend({
            typ.value: const.VALID_SETREQ[typ]
            for typ in const.VALID_TYPES[self.type]})