dm: rtc: Correct rtc_read32() return value

The current check is incorrect and will fail when any non-zero byte is read.
Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

drivers/rtc/rtc-uclass.c

@@ -68,7 +68,7 @@ int rtc_read32(struct udevice *dev, unsigned int reg, u32 *valuep)
 
 	for (i = 0; i < sizeof(value); i++) {
 		ret = rtc_read8(dev, reg + i);
-		if (ret)
+		if (ret < 0)
 			return ret;
 		value |= ret << (i << 3);
 	}