kern/53930: usmsc: detachcv is not initialized

[sc dying <sc.dying%gmail.com@localhost>]

>Number:         53930
>Category:       kern
>Synopsis:       usmsc: detachcv is not initialized
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Feb 01 09:20:00 +0000 2019
>Originator:     sc dying <sc.dying%gmail.com@localhost>
>Release:        NetBSD 8.99.32
>Organization:
>Environment:
System: NetBSD 8.99.32 evbarm earmv7hf
Architecture: earmv7hf
Machine: evbarm
>Description:
sc->sc_detachcv of usmsc(4) is not initialized.

>How-To-Repeat:
`drvctl -d usmsc0' panics my RPI definitely.

>Fix:
Please cv_init sc->sc_detachcv in smsc_attach.