qcacld-2.0: Update correct value for low power stats

In the present scenario the value that is being assigned to the
effective total on time is incorrect. The cumulative_total_on_time_ms
is being assigned which does not take into account the sleep time

Subtract the sleep time from the on time to reflect the correct
on time

Change-Id: I8a9738b1a9592ca182fb93e503a49d6ca336154d
CRs-Fixed: 2192936