hvsi: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.
Directly cancel hp->writer and flush hp->handshaker instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>