s390/qeth: Remove redundant 'flush_workqueue()' calls

'destroy_workqueue()' already drains the queue before destroying it, so
there is no need to flush it explicitly.

Remove the redundant 'flush_workqueue()' calls.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Acked-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://lore.kernel.org/r/20220216075155.940-1-vulab@iscas.ac.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>