usb: cdc_acm: fix bug on transmission

In the function tx_work_handler, a pointer to ring buffer data is given
to usb_transfer then the ring_buf_get_finish is called. So, the data is
mark as read (by ring_buf_get_finish) while the data are not yet
transferred to usb. If later a user send data, the pointer hold by usb
stack could be rewrite.

Signed-off-by: Julien D'Ascenzio <julien.dascenzio@paratronic.fr>