staging: drm/imx: fix return value check in imx_drm_init()

In case of error, the function platform_device_register_simple() 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>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/imx-drm/imx-drm-core.c

@@ -852,8 +852,8 @@ static int __init imx_drm_init(void)
 	INIT_LIST_HEAD(&imx_drm_device->encoder_list);
 
 	imx_drm_pdev = platform_device_register_simple("imx-drm", -1, NULL, 0);
-	if (!imx_drm_pdev) {
-		ret = -EINVAL;
+	if (IS_ERR(imx_drm_pdev)) {
+		ret = PTR_ERR(imx_drm_pdev);
 		goto err_pdev;
 	}