DSPLLE Thread: Fix a possible desync issue

It might be possible for the cpu thread call:
Common::AtomicStore(m_cycle_count, dsp_cycles);
and
dspEvent.Set();
faster than the dsp thread calls:
dspEvent.Wait();

When/if that happens, the current set of dsp cycles will not be processed, and instead the dsp thread will wait for the next set.