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>
diff --git a/drivers/rtc/rtc-opal.c b/drivers/rtc/rtc-opal.c
index 60f2250..3dd9d26 100644
--- a/drivers/rtc/rtc-opal.c
+++ b/drivers/rtc/rtc-opal.c
@@ -224,7 +224,7 @@
 	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 };