SH: cpuidle: check error code at init

Registering the driver, or the device, can fail, let's check the return code
and return the error code to the PM layer.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>