bluetooth: make monitor timestamps roll over less

With e.g. a 216MHz core clock, the 32-bit cycle counter overflows after
just 20 seconds. The 32-bit monitor timestamp (100us resolution) overflows
after around five days regardless of what we do, but we should try our best
to reach that.

Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>