dmaengine: pl330: Clear callback_result for re-used descs

The pl330 driver re-uses DMA descriptors rather than reallocating
them each time. At present, upon re-use the .callback member is
cleared, but .callback result is not. This causes problems where a
consuming driver sets the .callback_result for some submissions but
not for others, as eventually the function is invoked erronously.

Clear .callback_result along with .callback

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20240118112959.1027471-1-dan.scally@ideasonboard.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>