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>