fix use correct buffer size

lib/usb_cdcacm.c

@@ -326,7 +326,7 @@ static void cdcacm_data_tx_cb(usbd_device *usbd_dev, uint8_t ep)
 		// while(usbd_ep_write_packet(usb_device, 0x82, usb_data, usb_length)==0);
 		// this is less critical
 		uint8_t transmitted = usbd_ep_write_packet(usb_device, 0x82, usb_data, usb_length); // try to transmit data
-		tx_i = (tx_i+transmitted)%sizeof(rx_buffer); // update location on buffer
+		tx_i = (tx_i+transmitted)%sizeof(tx_buffer); // update location on buffer
 		tx_used -= transmitted; // update used size
 	}
 }