dm: rtc: Rename to_tm() to rtc_to_tm() and add error code

Rename this function so that it is clear that it is provided by the RTC.
Also return an error when it cannot function as expected. This is unlikely
to occur since it works for dates since 1752 and many RTCs do not support
such old dates. Still it is better to be accurate.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
diff --git a/drivers/rtc/date.c b/drivers/rtc/date.c
index 2000565..79beb94 100644
--- a/drivers/rtc/date.c
+++ b/drivers/rtc/date.c
@@ -71,7 +71,7 @@
 	return 0;
 }
 
-void to_tm(int tim, struct rtc_time * tm)
+int rtc_to_tm(int tim, struct rtc_time *tm)
 {
 	register int    i;
 	register long   hms, day;
@@ -103,10 +103,14 @@
 	/* Days are what is left over (+1) from all that. */
 	tm->tm_mday = day + 1;
 
+	/* Zero unused fields */
+	tm->tm_yday = 0;
+	tm->tm_isdst = 0;
+
 	/*
 	 * Determine the day of week
 	 */
-	rtc_calc_weekday(tm);
+	return rtc_calc_weekday(tm);
 }
 
 /* Converts Gregorian date to seconds since 1970-01-01 00:00:00.