4.20.7: pl2303 not working (post-4.19 regression) (limited info so far, not yet bisected)

[Nix]

So I just tried to connect up to my ancient Soekris firewall's serial
console to try to bisect a problem where it stopped booting in 4.20, and
found I couldn't.

minicom says:

minicom: cannot open /dev/ttyUSB0: Input/output error

and in the dmesg we see

[705576.028170] pl2303 ttyUSB0: failed to submit interrupt urb: -28

Booting to 4.19, everything works fine. (A random GalliumOS Chromebook
running 4.9.4 works fine too, not that that confirmation is terribly
useful.)

This is an extremely preliminary report in case it's instantly obvious
what's going on: I'll do enough investigation to produce an actually
useful bug report, including bisecting this, after I've bisected the
*other* non-booting bug, but that might not be until next weekend. (All
this for a firewall I was trying to decommission! bah :) )