dmaengine: bcm2835-dma: Use vchan_terminate_vdesc() instead of desc_free

To avoid race with vchan_complete, use the race free way to terminate
running transfer.

Implement the device_synchronize callback to make sure that the terminated
descriptor is freed.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>