Removed second division by refdiv

[vinsynth]

While setting up a custom clock with refdiv = 2, I noticed it's post division frequency was half it's expected value; it seems that when making a new PhaseLockedLoopDevice, division of the reference frequency by refdiv occurs twice, which I assume is unintentional.

[9names]

It's not obvious to me where this second division by refdiv occurs, would you mind pointing it out?

[jannic]

@9names: A few lines above, ref_freq_hz is calculated as:

        let ref_freq_hz: HertzU32 = xosc_frequency
            .to_Hz()
            .checked_div(u32::from(config.refdiv))
            .ok_or(Error::BadArgument)?
            .Hz();

So that value is already xosc_frequency / refdiv. Dividing it again looks wrong.

The patch LGTM.
(Of course, rustfmt should be applied.)