kern/51111: USB wifi device overheating issues (?)

[coypu%sdf.org@localhost]

>Number:         51111
>Category:       kern
>Synopsis:       USB wifi device overheating issues (?)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue May 03 01:25:00 +0000 2016
>Originator:     coypu
>Release:        NetBSD-7.99.29
>Organization:
>Environment:
NetBSD rpi 7.99.29 NetBSD 7.99.29 (RPI.201604292140Z) evbarm
>Description:

>How-To-Repeat:
1. connect a EW-7811UN device (very small USB wifi chip - prone to heat issues), which is supported by urtwn driver.
2. run wpa_supplicant & dhcpcd, wait a while*.

device will overheat and stop functioning, but netbsd doesn't notice anything**.

device still seems to receive power in some manner that it stays hot.
the only 'recovery' is manual removal of the USB stick and plugging it back in.
LED stays on.

* even if no network traffic is happening
** wpa_supplicant still reports the last event - that it connected successfully, no new dmesg messages, etc.
>Fix: