uss: signed/unsigned char fallout

uss: signed/unsigned char fallout

When char is signed, assigning 255 to a variable of type char changes
the value, which causes clang to emit a warning and fail the
--enable-checking build.

Change-Id: Id02e2526a9a9dd6657dee55b9dc22da03d102d8c
Reviewed-on: https://gerrit.openafs.org/13475
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>