drivers: soc: qcom: lpm-stats: Fix negative sleep times

A race between last core entering cluster mode and first core exiting
it could result in negative sleep times. This is stored into a uint
resulting in large sleep values for cluster modes.

Record negative sleep times as failed low power modes, as there
wouldn't have been any time for the cluster to execute any low power
mode.

Change-Id: I8924a2e90b8fd7b44533bed74c7ca0a74dee81fe
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>