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/cpu/s3c44b0/cpu.c b/cpu/s3c44b0/cpu.c
index eae6adb..fd09bf9 100644
--- a/cpu/s3c44b0/cpu.c
+++ b/cpu/s3c44b0/cpu.c
@@ -188,7 +188,7 @@
 	return 0;
 }
 
-void rtc_set (struct rtc_time* tm)
+int rtc_set (struct rtc_time* tm)
 {
 	if(tm->tm_year < 2000)
 		tm->tm_year -= 1900;
@@ -204,6 +204,8 @@
 	BCDMIN = HEX2BCD(tm->tm_min);
 	BCDSEC = HEX2BCD(tm->tm_sec);
 	RTCCON &= 1;
+
+	return 0;
 }
 
 void rtc_reset (void)