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