serial_cs: drop spurious flush_scheduled_work() call

serial_cs doesn't use the system workqueue.  Drop spurious
flush_scheduled_work() call.

This is to prepare for the deprecation and removal of
flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>