char: xillybus: Don't destroy workqueue from work item running on it

In the Linux kernel, the following vulnerability has been resolved:

char: xillybus: Don't destroy workqueue from work item running on it

Triggered by a kref decrement, destroy_workqueue() may be called from

within a work item for destroying its own workqueue. This illegal

situation is averted by adding a module-global workqueue for exclusive

use of the offending work item. Other work items continue to be queued

on per-device workqueues to ensure performance.