PM/devfreq: Fix uninitialized use of stat variable

Initialize stat variable to zero to avoid uninitialized use of
stat.private_data in the devfreq_vbif_ev_handler().

Change-Id: Ie34fad548214f287b67a8413c8658ebac65a95d3
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>