It seems the APIs don't cover your needs exactly. But I suggest you may try the following function:
void wiced_hal_puart_synchronous_write ( UINT8 * buffer, UINT32 length)
It is a blocking function and it makes sure your data will go into FIFO before returning from it. So I think It can help you with what you need.
Thank you for your suggestion.
I tried wiced_hal_puart_synchronous_write().
But unfortunately I find that this function is not blocked.
If I change baudrate by using wiced_hal_puart_configuration() after issuing wiced_hal_puart_synchronous_write(), baudrate is changed during data transmitting.