dmaengine: pl330: Embed pl330_req directly into dma_pl330_desc

The pl330_req struct is embedded into the dma_pl330_desc struct. But half of the
pl330_req struct are pointers to other fields of the dma_pl330_desc struct it is
embedded to. By directly embedding the fields from the pl330_req struct into the
dma_pl330_desc struct and reworking the code to work with the dma_pl330_desc
struct those pointers can be eliminated. This slightly simplifies the code.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>