PERF_ITRACE_PERIOD_INSTRUCTIONS is zero so it got overwritten by the
default period type.

Fix by checking if the period type was set rather than if the value was
zero when applying the default.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1437150840-31811-12-git-send-email-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>