cpuidle: fix improper return value on error

In function cpuidle_add_state_sysfs(), variable ret takes the return
value. Its value should be negative on errors. Because ret is reset in
the loop, its value will be 0 during the second and after repeat of the
loop. If kzalloc() returns a NULL pointer then, it will return 0. It may
be better to explicitly assign "-ENOMEM" when the call to kzalloc()
fails.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=188901
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>