iio: light: tsl2563: Remove flush_scheduled_work

flush_scheduled_work is scheduled for deprecation.
Replace cancel_delayed_work and flush_scheduled_work with
cancel_delayed_work_sync instead to ensure there is no pending or
running work item.

Since there is only one work item, chip->poweroff_work, there are
no further dependencies of flush_scheduled_work().

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>