isdn/capi: make kcapi use a separate workqueue

flush_scheduled_work() is deprecated and will be removed.  Because
kcapi uses fire-and-forget type works, it's impossible to flush each
work explicitly.  Create and use a dedicated workqueue instead.

Please note that with recent workqueue changes, each workqueue doesn't
reserve a lot of resources and using it as a flush domain is fine.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jan Kiszka <jan.kiszka@web.de>