dmaengine: st_fdma: Fix the error return code in st_fdma_probe()

In case of error, the function st_slim_rproc_alloc() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/st_fdma.c

@@ -792,7 +792,7 @@ static int st_fdma_probe(struct platform_device *pdev)
 	}
 
 	fdev->slim_rproc = st_slim_rproc_alloc(pdev, fdev->fw_name);
-	if (!fdev->slim_rproc) {
+	if (IS_ERR(fdev->slim_rproc)) {
 		ret = PTR_ERR(fdev->slim_rproc);
 		dev_err(&pdev->dev, "slim_rproc_alloc failed (%d)\n", ret);
 		goto err;