cdcacm.c: Use less buffer for the usb_uart buffers and reallocate.

With 128 bytes for both usb_uart buffers, traceswo gives errors!
Keep the size for the receive buffer and diminisch the transmit buffer,
as there is no flow control to the device.
Probably related to https://github.com/libopencm3/libopencm3/issues/477

src/platforms/common/cdcacm.c

@@ -517,7 +517,7 @@ static void cdcacm_set_config(usbd_device *dev, uint16_t wValue)
 
 	/* Serial interface */
 	usbd_ep_setup(dev, 0x03, USB_ENDPOINT_ATTR_BULK,
-	              CDCACM_PACKET_SIZE, usbuart_usb_out_cb);
+	              CDCACM_PACKET_SIZE / 2, usbuart_usb_out_cb);
 	usbd_ep_setup(dev, 0x83, USB_ENDPOINT_ATTR_BULK,
 	              CDCACM_PACKET_SIZE, usbuart_usb_in_cb);
 	usbd_ep_setup(dev, 0x84, USB_ENDPOINT_ATTR_INTERRUPT, 16, NULL);