dmaengine: fix null ptr check for __dma_async_device_channel_register()

Add check to pointer after assignment before accessing members.

Fixes: d2fb0a04: ("dmaengine: break out channel registration")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/158049351973.45445.3291586905226032744.stgit@djiang5-desk3.ch.intel.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dmaengine.c

@@ -962,6 +962,9 @@ static int __dma_async_device_channel_register(struct dma_device *device,
 
 	tchan = list_first_entry_or_null(&device->channels,
 					 struct dma_chan, device_node);
+	if (!tchan)
+		return -ENODEV;
+
 	if (tchan->dev) {
 		idr_ref = tchan->dev->idr_ref;
 	} else {