defmt::assert gives a compile error in const context.

Ensure that the fractional part of the clock divider is accurately
calculated. This does additional u32 division/mod operations, which I
think is better than using u64 for its extra precision.

Also:
- Add additional asserts to catch out-of-bounds results.
- Make the version that accepts the system clock as a parameter
const and public, so if anyone wants to hardcode the system
frequency they can avoid any runtime computation.
- Remove the `inline` attributes because the function has grown quite a
bit.

The test will fail, because the current implementation doesn't calculate
the fractional part.