usb: change LIST to SLIST to avoid LinuxKPI conflicts

In order to better integrate modern LinuxKPI USB this tries to reduce
a contention point of "LIST". Given there is no need to use a LIST here
change it to SLIST to avoid conflicts.
It is a workaround which does not solve the actual problem (overlapping
namespaces) but it helps us a lot for now.

Sponsored by: The FreeBSD Foundation
X-MFC? unclear
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D46534