rtc: recycle id when unloading a rtc driver

When calling rtc_device_unregister, we are not freeing the id used by the
driver.  So when doing a unload/load cycle for a RTC driver (e.g.  rmmod
rtc_cmos && modprobe rtc_cmos), its id is incremented by one.  As a
consequence, we no longer have neither an rtc0 driver nor a
/proc/driver/rtc (as it only exists for the first driver).
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/class.c

@@ -238,6 +238,7 @@ void rtc_device_unregister(struct rtc_device *rtc)
 		rtc_proc_del_device(rtc);
 		device_unregister(&rtc->dev);
 		rtc->ops = NULL;
+		ida_simple_remove(&rtc_ida, rtc->id);
 		mutex_unlock(&rtc->ops_lock);
 		put_device(&rtc->dev);
 	}