Detection failure with 1.20.4

[Peter Naulls]

Attached is a log of a modem detection failure I have on my system.  This 
platform is pretty similar to my legacy platform, but there are some differences:

* Now running 5.10 series kernels on both.
* The board has WiFi, which means a lot more of the PCI subsystem gets exposed 
in user space, which may be relevant for udev etc.
* There's additional LAN ports and a WAN port - again, not directly relevant to 
modem detection, but might have some effects with udev too.
* Subtle differences in USB hardware setup. There's a "USB passthrough" mode on
this board, and sometimes this seems to misbehave. In one instance I saw the 
modem reported twice in lsusb, although this seems to have been a one-off.

In this log, I've disabled PPP, and out of desperation, DHCP too, to limit
the churn.  This problem appears 100% of the time at start up, and is only
resolved by a restart of ModemManager, or a cycling of the USB interfaces
(which the top-level monitoring script will eventually do, but takes some 
minutes in practice).

The misdetection can happen on the legacy platform, but having disabled PPP
and extra plugins it seems to be pretty rare.

Let me know if there's extra debug I can provide.

Thanks!
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