If the --itrace option is used more than once, the options are
combined, but "i" and "y" (sub-)options can be corrupted because
itrace_do_parse_synth_opts() incorrectly overwrites the period type and
period with default values.

For example, with:

	--itrace=i0ns --itrace=e

The processing of "--itrace=e", resets the "i" period from 0 nanoseconds
to the default 100 microseconds.

Fix by performing the default setting of period type and period only if
"i" or "y" are present in the currently processed --itrace value.

Fixes: f6986c95 ("perf session: Add instruction tracing options")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240315071334.3478-2-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>