ARM: cpuidle: Fix error return code

We know that 'ret = 0' because it has been tested a few lines above.
So, if 'kzalloc' fails, 0 will be returned instead of an error code.
Return -ENOMEM instead.

Fixes: a0d46a3d ("ARM: cpuidle: Register per cpuidle device")
Signed-off-by: Christophe Jaillet <christophe.jaillet@wanadoo.fr>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: 4.1+ <stable@vger.kernel.org> # 4.1+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>