dmaengine: xilinx: dpdma: Fix kerneldoc warning

Document the struct xilinx_dpdma_chan desc field to fix a kerneldoc
undocumented member warning (which can be reproduced by compiling with
W=1).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20200722161747.30048-1-laurent.pinchart@ideasonboard.com


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