clockintr_advance: tweak logic to eliminate early-return

clockintr_advance: tweak logic to eliminate early-return

With the switch to clockintr_schedule_locked(), clockintr_advance() is
now much shorter and the early-return from the non-mutex path doesn't
make the function any easier to read.  Move the mutex path into the else
branch and always return 'count' at the end of the function.