Fix a bug introduced in the last commit. When calculating the delta count

Fix a bug introduced in the last commit.  When calculating the delta count
from the 8254 we have to use timer0_max_count + 1 instead of timer0_max_count
because our frequency correction may load timer0_max_count + 1.

Implement clkintr_pending in the FAST_INTR path as well as the normal INTR
path.