tools: cpupower: Fix error when running cpupower monitor

get_cpu_topology() tries to get topology info from all cpus by reading
files in the topology sysfs dir. If a cpu is offlined, since it doesn't
have topology dir, this function fails and returns -1. This causes
functions relying on get_cpu_topology() to fail. For example-

$ cpupower monitor
Cannot read number of available processors

Fix this by skipping fetching topology info for offline cpus.

Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Reported-by: Pavaman Subramaniyam <pavsubra@linux.vnet.ibm.com>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>