better handle missing I2C muxes

[bcantrill]

Draft commit comment:

After the fix to #1413, the system is unwilling to ignore mux related
errors -- which has generated a problem on some lab systems that are
missing muxes:  because we can never get the bus into a known mux
state these systems now become entirely unusable, as even devices that
aren't on the missing segments are not accessible.  This fixes that by
observing that a mux that is affirmatively missing -- that is, one
that doesn't reply to its in-band management at all -- can be assumed
to have segments that are similarly missing (and therefore as good as
disabled).  This (naturally) doesn't change the fact that accessing
any devices attached to the missing mux will generate an error, but it
allows the system to broadly drive on absent accesses to those
devices.

[cbiffle]

Huh, this diff is way smaller than I was expecting. So the key is just permitting a single error condition on the enable_segment path?

[cbiffle]

Nit:

Suggested change

	Err(code) if code == ResponseCode::MuxMissing => {
	Err(ResponseCode::MuxMissing) => {

[cbiffle]

I think these lines are equivalent to other => other, fwiw.

[bcantrill]

🤯🤯🤯 HOW LONG HAS THAT BEEN THERE?!

[bcantrill]

That is a really useful pattern (ha!) that I basically assumed (for terrible reasons?) didn't/wouldn't work.

[bcantrill]

Huh, this diff is way smaller than I was expecting. So the key is just permitting a single error condition on the enable_segment path?

Yeah -- the other paths actually handle themselves, it turns out. When we treat a missing mux as implying that all of its segments are in fact disabled, everything else just... works.