Cancel each work properly and remove flash_work_queue.

Quoting documentation:

In most situations flushing the entire workqueue is overkill; you merely
need to know that a particular work item isn't queued and isn't running.
In such cases you should use cancel_delayed_work_sync() or
cancel_work_sync() instead.
Signed-off-by: NTomas Winkler <tomas.winkler@intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>