Input: hp680_ts_input - use cancel_delayed_work_sync()

Make hp680_ts_init/exit() call cancel_delayed_work_sync() instead of
calling cancel_delayed_work() followed by flush_scheduled_work().

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

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>