dmaengine: edma: Fetch echan->edesc while holding lock in edma_comletion_handler

In order to avoid possible race condition when client drivers are using
dmaengine_terminate_sync() call to disable the channel.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>