rtc: opal: Make opal_tpo_alarm_irq_enable static

Fix sparse warning:

drivers/rtc/rtc-opal.c:227:5:
 warning: symbol 'opal_tpo_alarm_irq_enable' was not declared. Should it be static?
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-opal.c

@@ -224,7 +224,7 @@ static int opal_set_tpo_time(struct device *dev, struct rtc_wkalrm *alarm)
 	return rc;
 }
 
-int opal_tpo_alarm_irq_enable(struct device *dev, unsigned int enabled)
+static int opal_tpo_alarm_irq_enable(struct device *dev, unsigned int enabled)
 {
 	struct rtc_wkalrm alarm = { .enabled = 0 };