tools: cpupower: fix return checks for sysfs_get_idlestate_count()

Red Hat and Fedora use a bug reporting tool that gathers data about
"broken" systems called sosreport.  Among other things, it includes the
output of 'cpupower idle-info'.  Executing 'cpupower idle-info' on a
system that has cpuidle disabled via 'cpuidle.off=1' results in a 300
second hang in the cpupower application.

ie)
[root@intel-brickland-05]# cpupower idle-info
Could not determine cpuidle driver

Analyzing CPU 0:
Number of idle states: -19
[hang]

The problem is that the cpupower code only checks for a zero return from
sysfs_get_idlestate_count().  The function can return -ENODEV (-19) as
above.  This patch fixes callers to sysfs_get_idlestate_count() to check
the right return values.

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