staging: greybus: 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.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Minghao Chi (CGEL ZTE) <chi.minghao@zte.com.cn>
Link: https://lore.kernel.org/r/20220210060205.1607792-1-chi.minghao@zte.com.cn


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