RTC: Prevents a division by zero in kernel code.

This patch prevents a user space program from calling the RTC_IRQP_SET
ioctl with a negative value of frequency. Also, if this call is make
with a zero value of frequency, there would be a division by zero in the
kernel code.

[jstultz: Also initialize irq_freq to 1 to catch other divbyzero issues]

CC: Alessandro Zummo <[e-mail]>
CC: Thomas Gleixner <[e-mail]>
Signed-off-by: Marcelo Roberto Jimenez <[e-mail]>
Signed-off-by: John Stultz <[e-mail]>

drivers/rtc/class.c

···
 	rtc->id = id;
 	rtc->ops = ops;
 	rtc->owner = owner;
+	rtc->irq_freq = 1;
 	rtc->max_user_freq = 64;
 	rtc->dev.parent = dev;
 	rtc->dev.class = rtc_class;

drivers/rtc/interface.c

···
 	int err = 0;
 	unsigned long flags;
 
+	if (freq <= 0)
+		return -EINVAL;
+
 	spin_lock_irqsave(&rtc->irq_task_lock, flags);
 	if (rtc->irq_task != NULL && task == NULL)
 		err = -EBUSY;