tools / cpupower: Fix no idle state information return value

sysfs_get_idlestate_count() returns an unsigned int.  Returning -ENODEV
is not the right thing to do here, and in any case is handled the same
way as if there are no states found.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>