usb: cdc_acm: rework cdc_acm_poll_out to non-blocking

Change cdc_acm_poll_out to do the best to mimic behavior
of a hardware UART controller without flow control.

With this patch, if the USB subsystem is not ready,
no data is transfered to the buffer, that is, new character
is dropped. If the USB subsystem is ready and the buffer is full,
the first character from the tx_ringbuf is removed to
make room for the new character.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>