Windows: AFSObjectInfoDecrement correct return value

The return value must be the new ObjectReferenceCount value not
the value of the Reason counter.  This was broken by patchset
e267774f1a4f9cc5bfc32c545df36a4bf7947607.

Change-Id: I9dd3cd56f6c7d43f061bdda8e24666b5f9a4ed9b
Reviewed-on: http://gerrit.openafs.org/9068
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>