[media] cx25821: Drop Freeing of Workqueue

Workqueues shouldn't be freed. destroy_workqueue should be used instead.
destroy_workqueue safely destroys a workqueue and ensures that all pending
work items are done before destroying the workqueue.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>