rtc: mpc5121: Use correct return value for mpc5121_rtc_probe()

When call function devm_platform_ioremap_resource(), we should use IS_ERR()
to check the return value and return PTR_ERR() if failed.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1590370788-15136-2-git-send-email-yangtiezhu@loongson.cn

drivers/rtc/rtc-mpc5121.c

@@ -316,7 +316,7 @@ static int mpc5121_rtc_probe(struct platform_device *op)
 	rtc->regs = devm_platform_ioremap_resource(op, 0);
 	if (IS_ERR(rtc->regs)) {
 		dev_err(&op->dev, "%s: couldn't map io space\n", __func__);
-		return -ENOSYS;
+		return PTR_ERR(rtc->regs);
 	}
 
 	device_init_wakeup(&op->dev, 1);