dmaengine: at_hdmac: Introduce atc_get_llis_residue()

Introduce a method to get the residue for a hardware linked list transfer.
It makes the code easier to read.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20221025090306.297886-1-tudor.ambarus@microchip.com
Link: https://lore.kernel.org/r/20221025090306.297886-22-tudor.ambarus@microchip.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>