dmaengine: stm32-dmamux: set dmamux channel id in dma features bitfield

STM32 DMAMUX is used with STM32 DMA1 and DMA2:
- DMAMUX channels 0 to 7 are connected to DMA1 channels 0 to 7
- DMAMUX channels 8 to 15 are connected to DMA2 channels 0 to 7

STM32 MDMA can be triggered by DMA1 and DMA2 channels transfer complete,
and the "request line number" is the DMAMUX channel id (e.g. DMA2 channel 0
triggers MDMA with request line 8).

To well configure MDMA, set DMAMUX channel id in DMA features bitfield,
so that DMA can update struct dma_slave_config peripheral_config properly.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20220829154646.29867-5-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>