hw/cxl: rename mailbox return code type from ret_code to CXLRetCode

hw/cxl: rename mailbox return code type from ret_code to CXLRetCode

Given the increasing usage of this mailbox return code type, now
is a good time to switch to QEMU style naming.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20230423162013.4535-2-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>