perf tools: Unset perf_event_attr::freq when period term is set

We need to unset 'perf_event_attr::freq' bit (default 1) when
'period' term is specified within event definition like:

  -e 'cpu/cpu-cycles,call-graph=fp,time,period=100000'

otherwise it will handle the period value as frequency
(and fail if it crossed the maximum allowed frequency value).

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20150808171210.GC17040@krava.brq.redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>