dmaengine: rcar-dmac: Fix oops due to unintialized list in error ISR

The error interrupt handler stops and reinitializes all channels. This
causes a crash for channels that have never been used, as their
descriptor lists are uninitialized. Fix it by initializing the
descriptor lists at probe time.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>