time: ntp: simplify ntp_tick_adj calculations

Impact: micro-optimization

Convert the (internal) ntp_tick_adj value we store from unscaled
units to scaled units. This is a constant that we never modify,
so scaling it up once during bootup is enough - we dont have to
do it for every adjustment step.

Signed-off-by: Ingo Molnar <mingo@elte.hu>