cpuidle: lpm-levels: Check for negative sleep times

A negative sleep time could be interpreted as a large positive sleep
time when its casted to a unsigned type. This could result in CPU sleeping
for a longer than expected duration resulting in missed interrupts.

Change-Id: I472f355c8ba392ab46d53fceddb448f7e35c178e
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>
Signed-off-by: Naresh Malladi <namall@codeaurora.org>