dmaengine: rcar-dmac: Fixed active descriptor initializing

dmaengine: rcar-dmac: Fixed active descriptor initializing

Running descriptor pointer is set to NULL upon freeing resources. Other-
wise, rcar_dmac_issue_pending might not start new transfers

Signed-off-by: Muhammad Hamza Farooq <mfarooq@visteon.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>