dwc_eth_qos: Remove deprecated create_singlethread_workqueue

alloc_workqueue replaces deprecated create_singlethread_workqueue().

A dedicated workqueue has been used since the workitem viz
lp->txtimeout_reinit is involved in reinitialization if a TX timeout
occurs, which is necessary to guarantee forward progress in packet
processing. As a network device can be used during memory reclaim, the
workqueue needs forward progress guarantee under memory pressure.
WQ_MEM_RECLAIM has been set to ensure this.

Since there is only a single work item, explicit concurrency limit is
unnecessary here.

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