ARM: EXYNOS: handle properly the return values

The cpuidle_register_driver return value is not checked.
The init function returns always -EIO when cpuidle_register_device
fails but the error could be different.

This patch fixes that by checking the cpuidle_register_driver properly
and returning the correct value when cpuidle_register_device fails.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>