mwl8k: mwl8k_queue_work() cleanup

Delete most of the mwl8k_work_struct fields and options, since most
of them are unused or never changed from their defaults:
- We always use priv->config_wq, so delete the wqueue argument from
  mwl8k_queue_work().
- MWL8K_WQ_SPIN and MWL8K_WQ_POST_REQUEST are never used, as all
  callers sleep for request completion, so sleep unconditionally.
- MWL8K_WQ_FREE_WORKSTRUCT is never used.
- MWL8K_WQ_TX_WAIT_EMPTY is always set, so assume it unconditionally.
- timeout_ms/txwait_attempts/tx_timeout_ms are never changed from
  their defaults, so just hardcode these in the workqueue worker.
- step is never used.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>