rtc: Fix RTC PIE frequency limit

Thomas earlier submitted a fix to limit the RTC PIE freq, but
picked 5000Hz out of the air. Willy noticed that we should
instead use the 8192Hz max from the rtc man documentation.

Cc: Willy Tarreau <w@1wt.eu>
Cc: stable@kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>

drivers/rtc/interface.c

@@ -708,7 +708,7 @@ int rtc_irq_set_freq(struct rtc_device *rtc, struct rtc_task *task, int freq)
 	int err = 0;
 	unsigned long flags;
 
-	if (freq <= 0 || freq > 5000)
+	if (freq <= 0 || freq > RTC_MAX_FREQ)
 		return -EINVAL;
 retry:
 	spin_lock_irqsave(&rtc->irq_task_lock, flags);

include/linux/rtc.h

@@ -97,6 +97,9 @@ struct rtc_pll_info {
 #define RTC_AF 0x20	/* Alarm interrupt */
 #define RTC_UF 0x10	/* Update interrupt for 1Hz RTC */
 
+
+#define RTC_MAX_FREQ	8192
+
 #ifdef __KERNEL__
 
 #include <linux/types.h>