dmaengine: dma-jz4780: Simplify jz4780_dma_desc_residue()

Simple cleanup, no changes to actual logic here.

Signed-off-by: Daniel Silsby <dansilsby@gmail.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>