perf, powerpc: fsl_emb: Restore setting perf_sample_data.period

Commit 6b95ed34 changed from
a struct initializer to perf_sample_data_init(), but the setting
of the .period member was left out.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: stable@kernel.org
Signed-off-by: Paul Mackerras <paulus@samba.org>