USB: cdc-wdm: fix memory leak

commit 2f338c8a1904e2e7aa5a8bd12fb0cf2422d17da4 upstream.

cleanup() is not called if the last close() comes after
disconnect(). That leads to a memory leak. Rectified
by checking for an earlier disconnect() in release()

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>