dmaengine: stm32-dma: configure next sg only if there are more than 2 sgs

dmaengine: stm32-dma: configure next sg only if there are more than 2 sgs

DMA operates in Double Buffer Mode (DBM) when the transfer is cyclic and
there are at least two periods.
When DBM is enabled, the DMA toggles between two memory targets (SxM0AR and
SxM1AR), indicated by the SxSCR.CT bit (Current Target).
There is no need to update the next memory address if two periods are
configured, as SxM0AR and SxM1AR are already properly set up before the
transfer begins in the stm32_dma_start_transfer() function.
This avoids unnecessary updates to SxM0AR/SxM1AR, thereby preventing
potential Transfer Errors. Specifically, when the channel is enabled,
SxM0AR and SxM1AR can only be written if SxSCR.CT=1 and SxSCR.CT=0,
respectively. Otherwise, a Transfer Error interrupt is triggered, and the
stream is automatically disabled.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20250624-stm32_dma_dbm_fix-v1-1-337c40d6c93e@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>