The dma buffer should be services that are used on top of peripherals. As such,
the usart driver should'nt directly use them, this is up to the user. The
multi-buffer has also been simplified since I was not satisfied with the
previous implementation
Transfer Complete is preferable to TX Empty when using a DMA since we could be
polling the bit between transfers. TXE would be 1 but the DMA would be
transfering another at the same time. TC takes the DMA into account
