Improve HPET comparator reprog to prevent intr-near-missing case

HPET intr-near-missing means if the current counter value is too close
to the comparator value to be reprogrammed the expected HPET intr may
be missing. Linux kernel uses a mininal 48-hpet-ticks(~3.5us) distance
to workaround this, but personal observation showed there is still
failure case while delta=3D0xba (~13.5us). So choosing 20us as the
MIN_DELTA_NS should be helpful to prevent near-missing from happening.

local_irq_save/restore were used to avoid disturbance. (+ 2) was used
as a final guard to avoid wrong judgement due to the real happened
near-missing case.

Signed-off-by: Wei Gang <gang.wei@intel.com>
Signed-off-by: Tian Kevin <kevin.tian@intel.com>

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