staging: unisys: visornic: correctly clean up device on removal

staging: unisys: visornic: correctly clean up device on removal

visornic_remove() is called to logically detach the visornic driver from a
visorbus-supplied device, which can happen either just prior to a
visorbus-supplied device disappearing, or as a result of an rmmod of
visornic.  Prior to this patch, logic was missing to properly clean up for
this removal, which was fixed via the following changes:

* A going_away flag is now used to interlock between device destruction and
  workqueue operations, protected by priv_lock.  I.e., setting
  going_away=true under lock guarantees that no new work items can get
  queued to the work queues.  going_away=true also short-circuits other
  operations to enable device destruction to proceed.

* Missing clean-up operations for the workqueues, netdev, debugfs entries,
  and the worker thread were added.

* Memory referenced from the visornic private devdata struct is now freed
  as part of devdata destruction.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>