staging: wlan-ng: Remove flush_scheduled_work

flush_scheduled_work() is scheduled for deprecation. Use
cancel_work_sync() instead to ensure that there is no pending or
running work item.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>