dmaengine: at_xdmac: Move the free desc to the tail of the desc list

Move the free desc to the tail of the list, so that the sequence of
descriptors is more track-able in case of debug. One would know which
descriptor should come next and could easier catch concurrency over
descriptors for example. virt-dma uses list_splice_tail_init() as well,
follow the core driver.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20211215110115.191749-7-tudor.ambarus@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>