addition overflow in LargeCmisOp::build_many()

[Nieuwejaar]

I'm seeing dendrite crash repeatedly on london with an addition overflow here:

https://github.com/oxidecomputer/dendrite/blob/main/dpd/src/transceivers/tofino_impl.rs#L3592

    fn build_many(
        page: P,
        offset: u8,
        len: u8,
    ) -> Result<Vec<Self>, ControllerError> {
        let end = offset + len;     <---

I added a log message which showed that both offset and len were 128, explaining the crash. I suspect we don't often see this crash because we don't typically run a debug build, but I'm surprised that we've never seen this before. Maybe there's a new transceiver on the system that triggers this condition?

Converting these to usize before doing the math addresses the issue.