usb: device_next: CDC ACM: Process TX on configuration enable

If CDC ACM uart side has TX enabled when the configuration gets enabled,
depending on fifo state following has to happen:
  * if the fifo is empty - trigger TX ready interrupt
  * if the fifo is not empty - queue TX data on IN endpoint

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>