dmaengine: omap-dma: Correct status reporting for memcpy

During mem copy both src and dst position moves at the same pace. Check the
dst position for progress reporting.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>