PSC's ReadVcap operation probably reports wrong voltage

According to ACAN-114, the MWOCP68 PSU's READ_VCAP register is "Bulk Capacitor Voltage Sensor Reading in Vdc", but there's some confusion about the data format. The doc says it's in the normal Linear11 format, but the PSC firmware interprets it as VOutMode(Unsigned): https://github.com/oxidecomputer/pmbus/blob/100d5c2a7a7d8ca92974f444e9b5af4e7a196438/src/mwocp68.ron#L55.

I investigated the READ_VCAP register on the newer MWOCP67 PSUs used by the Observer, and concluded that it's probably in Linear11 format. Specifically, the Linear11 interpretation gives a reading of 465V and the VOutMode interpretation gives a reading of 125V. According to @ericaasen, 465V is more likely to be correct.

So, I'm using Linear11 for READ_VCAP on the Observer's new PSUs. I suspect we should also use Linear11 for the PSC's old PSUs, but we should probably test that before changing it. The steps are to read a MWOCP68's READ_VCAP register using both interpretations and ask an EE which one looks correct. I don't know if we actually use READ_VCAP for anything, so maybe none of this matters very much.

Start with src/mwocp68.ron at the READ_VCAP definition and trace how the PSC interprets that register. Read a MWOCP68's READ_VCAP value using both Linear11 and VOutMode(Unsigned), then ask an EE which matches the PSU voltage; done means confirming the format and updating the interpretation if needed.