debug: tracing: Fix time returned by CPU stats module

Function cpu_stats_get_ns from CPU stats module returns
invalid time, because of unnecessary conversion into u32_t.
Higher bits of time value are removed during this conversion.
All time values are u64_t, so there is no need to convert anything.

Signed-off-by: Dominik Kilian <Dominik.Kilian@nordicsemi.no>