tty: serial: max3100: 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>
Link: https://lore.kernel.org/r/20220114085156.43041-1-vulab@iscas.ac.cn


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