dmaengine: at_hdmac: Remove unused member of at_dma_chan

The pointer to at_dma engine was never used, remove it.

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/20221025090306.297886-32-tudor.ambarus@microchip.com


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