net: ethernet: cavium: liquidio: request_manager: Remove create_workqueue

alloc_workqueue replaces deprecated create_workqueue().

A dedicated workqueue has been used since the workitem viz
(&db_wq->wk.work which maps to check_db_timeout) is involved
in normal device operation. WQ_MEM_RECLAIM has been set to guarantee
forward progress under memory pressure, which is a requirement here.
Since there are only a fixed number of work items, explicit concurrency
limit is unnecessary.

flush_workqueue is unnecessary since destroy_workqueue() itself calls
drain_workqueue() which flushes repeatedly till the workqueue
becomes empty.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>