alloc_workqueue replaces deprecated create_workqueue().

A dedicated workqueue has been used since the workitems are involved
in normal device operation. Workitems &priv->rx_work and &priv->tx_work,
map to w5100_rx_work and w5100_tx_work respectively and are involved in
receiving and transmitting packets. Forward progress under
memory pressure is a requirement here.

create_workqueue has been replaced with alloc_workqueue with max_active
as 0 since there is no need for throttling the number of active work
items.

Since the driver may be used in memory reclaim path,
WQ_MEM_RECLAIM has been set to guarantee forward progress.

flush_workqueue is unnecessary since destroy_workqueue() itself calls
drain_workqueue() which flushes repeatedly till the workqueue
becomes empty. Hence the call to flush_workqueue() has been dropped.
Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>