usb: 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.

This was generated with coccinelle:

@@
expression E;
@@
- 	flush_workqueue(E);
	destroy_workqueue(E);

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Peter Chen <peter.chen@kernel.or> # for chipidea part
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/563123a8117d6cafae3f134e497587bd2b8bb7f4.1636734453.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>