drivers/rtc/rtc-spear.c: fix use-after-free in spear_rtc_remove()

`config' is freed and is then used in the rtc_device_unregister() call,
causing a kernel panic.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Reviewed-by: Viresh Kumar <viresh.linux@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>