staging: greybus: arche-platform: fix device reference leak

Make sure to drop the device reference taken by of_find_device_by_node()
before returning from arche_platform_change_state().

Note that this code is expected to be removed, but let's fix up the leak
nonetheless.

Fixes: 886aba55

 ("greybus: arche-platform: Export fn to allow...")
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Vaibhav Hiremath <hvaibhav.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>