dmaengine: stm32-dma: add alternate REQ/ACK protocol management

dmaengine: stm32-dma: add alternate REQ/ACK protocol management

STM32 USART/UART is not managing correctly the default DMA REQ/ACK protocol
leading to possibly lock the DMA stream.
Default protocol consists in maintaining ACK signal up to the removal of
REQuest and the transfer completion.
In case of alternative REQ/ACK protocol, ACK de-assertion does not wait the
removal of the REQuest, but only the transfer completion.

This patch retrieves the need of the alternative protocol through the
device tree, and sets the protocol accordingly.
It also unwrap STM32_DMA_DIRECT_MODE_GET macro definition for consistency
with new STM32_DMA_ALT_ACK_MODE_GET macro definition.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20210624093959.142265-3-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>