dmaengine: xilinx: xdma: Better handling of the busy variable

The driver internal scatter-gather logic is:
* set busy to true
* start transfer
<irq>
  * set busy to false
  * trigger next transfer if any
    * set busy to true
</irq>

Setting busy to false in cyclic transfers does not make any sense and is
conceptually wrong. In order to ease the integration of additional
callbacks let's move this change to the scatter-gather path.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20231130111315.729430-4-miquel.raynal@bootlin.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>