drivers: cpuidle: lpm-levels: check for valid LPM stats

When DEBUG_FS is not defined, the cluster->stats may be initialized to
an error value. Check for valid stats pointer before updating the start
and end times.

Change-Id: I126766a017ebd9a84bd40f19ab3ff47877615429
Signed-off-by: Lina Iyer <ilina@codeaurora.org>