tools: cpupower: fix wrong err msg not supported vs not available

idlestates in sysfs are counted from 0.

This fixes a wrong error message.
Current behavior on a machine with 4 sleep states is:

cpupower idle-set -e 4
Idlestate 4 enabled on CPU 0

-----Wrong---------------------
cpupower idle-set -e 5
Idlestate enabling not supported by kernel
-----Must and now will be -----
cpupower idle-set -e 5
Idlestate 6 not available on CPU 0
-------------------------------

cpupower idle-set -e 6
Idlestate 6 not available on CPU 0

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