rtc: allow rtc_set to return an error and use it in cmd_date

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/drivers/rtc/ds3231.c b/drivers/rtc/ds3231.c
index d8cd47d..da8a3e6 100644
--- a/drivers/rtc/ds3231.c
+++ b/drivers/rtc/ds3231.c
@@ -134,7 +134,7 @@
 /*
  * Set the RTC
  */
-void rtc_set (struct rtc_time *tmp)
+int rtc_set (struct rtc_time *tmp)
 {
 	uchar century;
 
@@ -152,6 +152,8 @@
 	rtc_write (RTC_HR_REG_ADDR, bin2bcd (tmp->tm_hour));
 	rtc_write (RTC_MIN_REG_ADDR, bin2bcd (tmp->tm_min));
 	rtc_write (RTC_SEC_REG_ADDR, bin2bcd (tmp->tm_sec));
+
+	return 0;
 }