pbio/uartdev: disallow mode setting on motors

We currently do not support changing mode if it is already in combo mode. Doing so currently causes the runtime to hang since the mode change never completes.

This changes makes the error explicit and avoids the hang.

lib/pbio/src/uartdev.c

@@ -1130,6 +1130,11 @@ static pbio_error_t ev3_uart_set_mode_begin(pbio_iodev_t *iodev, uint8_t mode) {
     uartdev_port_data_t *port_data = PBIO_CONTAINER_OF(iodev, uartdev_port_data_t, iodev);
     pbio_error_t err;
 
+    // User mode change for motors is not supported
+    if (PBIO_IODEV_IS_FEEDBACK_MOTOR(iodev)) {
+        return PBIO_ERROR_NOT_SUPPORTED;
+    }
+
     err = ev3_uart_begin_tx_msg(port_data, LUMP_MSG_TYPE_CMD, LUMP_CMD_SELECT, &mode, 1);
     if (err != PBIO_SUCCESS) {
         return err;