usb: device_next: cdc_acm: set the thread name after initializing it

The cdc-acm workqueue thread name is set before initializing it right
now, which results in an empty thread name as apparently it gets cleared
by k_work_queue_init. Fix it by moving the thread name setting after the
workqueu is initialized and started.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>