net/smc: no need to flush smcd_dev's event_wq before destroying it

destroy_workqueue() already calls drain_workqueue(), which is a stronger
variant of flush_workqueue().

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>