hwclock: Reduce drift factor update period

hwclock: Reduce drift factor update period

To facilitate the calculation of 'cold' vs 'warm' Hardware Clock drift
factor the limit on the update period needs to be less than 8 hours.

4 hours should be enough drift to allow calculations that are not
grossly out of range.

For example, with a workstation that is shutdown every night the cold
drift factor can be significantly different than a drift factor based on
a 24 hour period.

Signed-off-by: J William Piggott <elseifthen@gmx.com>