PM / sleep: fix device reference leak in test_suspend

Make sure to drop the reference taken by class_find_device() after
opening the RTC device.

Fixes: 77437fd4 (pm: boot time suspend selftest)
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>