ARM: OMAP: CPUidle: Unregister drivere on device registration failure

If the CPUidle device registration fails for some reason, we should
unregister the driver on error path.

Fix the code accordingly. Also when at it, check of the driver registration
failure too.

Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>