libertas: Remove create_workqueue

alloc_workqueue replaces deprecated create_workqueue().

In if_sdio.c, the workqueue card->workqueue has workitem
&card->packet_worker, which is mapped to if_sdio_host_to_card_worker.
The workitem is involved in sending packets to firmware.
Forward progress under memory pressure is a requirement here.

In if_spi.c, the workqueue card->workqueue has workitem
&card->packet_worker, which is mapped to if_spi_host_to_card_worker.
The workitem is involved in sending command packets from the host.
Forward progress under memory pressure is a requirement here.

Dedicated workqueues have been used in both cases since the workitems
on the workqueues are involved in normal device operation with
WQ_MEM_RECLAIM set to gurantee forward progress under memory pressure.
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. Hence the calls to flush_workqueue() before
destroy_workqueue() have been dropped.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>