dmaengine: stm32-mdma: Fix a possible null-pointer dereference in stm32_mdma_irq_handler()

In stm32_mdma_irq_handler(), chan is checked on line 1368.
When chan is NULL, it is still used on line 1369:
    dev_err(chan2dev(chan), "MDMA channel not initialized\n");

Thus, a possible null-pointer dereference may occur.

To fix this bug, "dev_dbg(mdma2dev(dmadev), ...)" is used instead.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Fixes: a4ffb13c ("dmaengine: Add STM32 MDMA driver")
Link: https://lore.kernel.org/r/20190729020849.17971-1-baijiaju1990@gmail.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/stm32-mdma.c

@@ -1366,7 +1366,7 @@ static irqreturn_t stm32_mdma_irq_handler(int irq, void *devid)
 
 	chan = &dmadev->chan[id];
 	if (!chan) {
-		dev_err(chan2dev(chan), "MDMA channel not initialized\n");
+		dev_dbg(mdma2dev(dmadev), "MDMA channel not initialized\n");
 		goto exit;
 	}