tools/power/x86/intel-speed-select: Add missing free cpuset

During perf level change cpuset is allocated but not freed.
Add free_cpu_set() in success and failure path.

Although this is not an issue, as the program will exit after
processing of online/offline, but for completeness add the
free_cpu_set().

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>