dmaengine: virt-dma: use vchan_vdesc_fini() to free descriptors

vchan_dma_desc_free_list() basically open codes vchan_vdesc_fini() in its
loop body. Call it directly rather than duplicating the code.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191216105328.15198-7-s.hauer@pengutronix.de


Signed-off-by: Vinod Koul <vkoul@kernel.org>