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/ds1556.c b/drivers/rtc/ds1556.c
index f95f28e..7574626 100644
--- a/drivers/rtc/ds1556.c
+++ b/drivers/rtc/ds1556.c
@@ -120,7 +120,7 @@
 	return 0;
 }
 
-void rtc_set( struct rtc_time *tmp )
+int rtc_set( struct rtc_time *tmp )
 {
 	uchar reg_a;
 #ifdef RTC_DEBUG
@@ -146,6 +146,8 @@
 
 	/* unlock clock registers after read */
 	rtc_write( RTC_CONTROLA, ( reg_a  & ~RTC_CA_WRITE ));
+
+	return 0;
 }
 
 void rtc_reset (void)