rtc: sh: Fix invalid alarm warning for non-enabled alarm

BugLink: https://bugs.launchpad.net/bugs/1830176

[ Upstream commit 15d82d22 ]

When no alarm has been programmed on RSK-RZA1, an error message is
printed during boot:

    rtc rtc0: invalid alarm value: 2019-03-14T255:255:255

sh_rtc_read_alarm_value() returns 0xff when querying a hardware alarm
field that is not enabled.  __rtc_read_alarm() validates the received
alarm values, and fills in missing fields when needed.
While 0xff is handled fine for the year, month, and day fields, and
corrected as considered being out-of-range, this is not the case for the
hour, minute, and second fields, where -1 is expected for missing
fields.

Fix this by returning -1 instead, as this value is handled fine for all
fields.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/rtc/rtc-sh.c

@@ -455,7 +455,7 @@ static int sh_rtc_set_time(struct device *dev, struct rtc_time *tm)
 static inline int sh_rtc_read_alarm_value(struct sh_rtc *rtc, int reg_off)
 {
 	unsigned int byte;
-	int value = 0xff;	/* return 0xff for ignored values */
+	int value = -1;			/* return -1 for ignored values */
 
 	byte = readb(rtc->regbase + reg_off);
 	if (byte & AR_ENB) {