dmaengine: xilinx: xdma: Clarify the logic between cyclic/sg modes

We support both modes, but they perform totally different taks in the
interrupt handler. Clarify what shall be done in each case.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20231130111315.729430-3-miquel.raynal@bootlin.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>