dmaengine: ptdma: Move variable condition check to the first place and remove redundancy

The variable is dereferenced without first checking if it's null, leading
to the following warning: 'Variable dereferenced before check: desc.'

     drivers/dma/amd/ptdma/ptdma-dmaengine.c: pt_cmd_callback_work()
     warn: variable dereferenced before check 'desc'

Therefore, move the condition check for the 'desc' variable to the first
place and remove the redundant extra condition check.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/bfa0a979-ce9f-422d-92c3-34921155d048@stanley.mountain/
Fixes: 6565439 ("dmaengine: ptdma: Utilize the AE4DMA engine's multi-queue functionality")
Signed-off-by: Basavaraj Natikar <Basavaraj.Natikar@amd.com>
Link: https://lore.kernel.org/r/20250421114215.1687073-1-Basavaraj.Natikar@amd.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: basuamd

drivers/dma/amd/ptdma/ptdma-dmaengine.c

@@ -342,6 +342,9 @@ static void pt_cmd_callback_work(void *data, int err)
 	struct pt_dma_chan *chan;
 	unsigned long flags;
 
+	if (!desc)
+		return;
+
 	dma_chan = desc->vd.tx.chan;
 	chan = to_pt_chan(dma_chan);
 
@@ -355,16 +358,14 @@ static void pt_cmd_callback_work(void *data, int err)
 		desc->status = DMA_ERROR;
 
 	spin_lock_irqsave(&chan->vc.lock, flags);
-	if (desc) {
-		if (desc->status != DMA_COMPLETE) {
-			if (desc->status != DMA_ERROR)
-				desc->status = DMA_COMPLETE;
+	if (desc->status != DMA_COMPLETE) {
+		if (desc->status != DMA_ERROR)
+			desc->status = DMA_COMPLETE;
 
-			dma_cookie_complete(tx_desc);
-			dma_descriptor_unmap(tx_desc);
-		} else {
-			tx_desc = NULL;
-		}
+		dma_cookie_complete(tx_desc);
+		dma_descriptor_unmap(tx_desc);
+	} else {
+		tx_desc = NULL;
 	}
 	spin_unlock_irqrestore(&chan->vc.lock, flags);