[S390] cio: move device unregistration to dedicated work queue

Use dedicated slow path work queue when unregistering a device due to
a user action. This ensures serialialization of other register/
unregister requests.

Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>