video: fbdev: imxfb: fix return value check in imxfb_probe()

If devm_ioremap_resource() fails, it never return NULL, replace
NULL test with IS_ERR().

Fixes: b083c22d5114 ("video: fbdev: imxfb: Convert request_mem_region + ioremap to devm_ioremap_resource")
Signed-off-by: Yang Yingliang <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/video/fbdev/imxfb.c

···
 	}
 
 	fbi->regs = devm_ioremap_resource(&pdev->dev, res);
-	if (fbi->regs == NULL) {
+	if (IS_ERR(fbi->regs)) {
 		dev_err(&pdev->dev, "Cannot map frame buffer registers\n");
-		ret = -ENOMEM;
+		ret = PTR_ERR(fbi->regs);
 		goto failed_ioremap;
 	}