rtc: snvs: fix an incorrect check of return value

Function devm_regmap_init_mmio() returns an ERR_PTR on error. However,
in function snvs_rtc_probe() its return value is checked against NULL.
This patch fixes it by checking the return value with IS_ERR().
Signed-off-by: NPan Bian <bianpan2016@163.com>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-snvs.c

@@ -258,7 +258,7 @@ static int snvs_rtc_probe(struct platform_device *pdev)
 		of_property_read_u32(pdev->dev.of_node, "offset", &data->offset);
 	}
 
-	if (!data->regmap) {
+	if (IS_ERR(data->regmap)) {
 		dev_err(&pdev->dev, "Can't find snvs syscon\n");
 		return -ENODEV;
 	}