dmaengine: rcar-dmac: Fix residue reporting for pending descriptors

Cookies corresponding to pending transfers have a residue value equal to
the full size of the corresponding descriptor. The driver miscomputes
that and uses the size of the active descriptor instead. Fix it.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[geert: Also check desc.active list]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>