staging: octeon: Convert create_singlethread_workqueue()

With conccurency managed workqueues, use of dedicated workqueues can
be replaced by system_wq. Drop cvm_oct_poll_queue by using system_wq.

There are multiple work items per cvm_oct_poll_queue (viz.
cvm_oct_rx_refill_work, port_periodic_work) and different
cvm_oct_poll_queues need not be be ordered. Hence, concurrency
can be increased by switching to system_wq.

All work items are sync canceled in cvm_oct_remove() so it
is guaranteed that no work is in flight by the time exit path runs.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>