Move time math out of disabled interrupts sections.
We don't need the result before next sleep time, so no reason to additionally increase interrupt latency. While there, remove extra PM ticks to microseconds conversion, making C2/C3 sleep times look 4 times smaller than really. The conversion is already done by AcpiGetTimerDuration(). Now I see reported sleep times up to 0.5s, just as expected for planned 2 wakeups per second. MFC after: 1 month