With a large enough HSE input frequency, the vco clock calculation will
overflow a u32. Therefore, in this specific case we have to use the
inner value and cast to u64 to ensure the mul isn't clipped before
applying the divider.
This makes the configuration more flexible and closer to the underlying
configuration register structure. For example, we could use HSI for the
system clock, but use HSE to output a clock with MCO.
