dmaengine: virt-dma: Fix access after free in vchan_complete()

vchan_vdesc_fini() is freeing up 'vd' so the access to vd->tx_result is
via already freed up memory.

Move the vchan_vdesc_fini() after invoking the callback to avoid this.

Fixes: 09d5b702

 ("dmaengine: virt-dma: store result on dma descriptor")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20191220131100.21804-1-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>