dmaengine: xilinx: dpdma: Fix race condition in done IRQ

The active descriptor pointer is accessed from different contexts,
including different interrupt handlers, and its access must be protected
by the channel's lock. This wasn't done in the done IRQ handler. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20210307040629.29308-3-laurent.pinchart@ideasonboard.com


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