afsmonitor: Allow CBSTATS collection to work

The switch which selects the collection number was missing a
'break', so selecting the CBSTATS collection would always fall
through to the default, error, case.

Caught by clang-analyzer

Change-Id: I354873d711c5f49ddad51f1b9fb332af91a6d13a
Reviewed-on: http://gerrit.openafs.org/9235
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>