drm: sti: Fix return value check in sti_drm_platform_probe()

In case of error, the function platform_device_register_resndata()
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 <yongjun_wei@trendmicro.com.cn>

drivers/gpu/drm/sti/sti_drm_drv.c

@@ -201,8 +201,8 @@ static int sti_drm_platform_probe(struct platform_device *pdev)
 	master = platform_device_register_resndata(dev,
 			DRIVER_NAME "__master", -1,
 			NULL, 0, NULL, 0);
-	if (!master)
-		return -EINVAL;
+	if (IS_ERR(master))
+               return PTR_ERR(master);
 
 	platform_set_drvdata(pdev, master);
 	return 0;