char: xillybus: Refine workqueue handling

As the wakeup work item now runs on a separate workqueue, it needs to be
flushed separately along with flushing the device's workqueue.

Also, move the destroy_workqueue() call to the end of the exit method,
so that deinitialization is done in the opposite order of
initialization.

Fixes: ccbde4b1 ("char: xillybus: Don't destroy workqueue from work item running on it")
Cc: stable <stable@kernel.org>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20240816070200.50695-1-eli.billauer@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>