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 <[e-mail]>
Acked-by: Peter Griffin <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/st_fdma.c

···
 	}
 
 	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;