dmaengine: rcar-dmac: Fix uninitialized variable usage

The desc variable is used uninitialized in the rcar_dmac_desc_get() and
rcar_dmac_xfer_chunk_get() functions if descriptors need to be
allocated. Fix it.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>