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/include/rtc.h b/include/rtc.h
index df2d35f..785fbe3 100644
--- a/include/rtc.h
+++ b/include/rtc.h
@@ -53,7 +53,7 @@
 };
 
 int rtc_get (struct rtc_time *);
-void rtc_set (struct rtc_time *);
+int rtc_set (struct rtc_time *);
 void rtc_reset (void);
 
 void GregorianDay (struct rtc_time *);