mwifiex: remove cfg_workqueue

cfg_workqueue was added to notify cfg80211 that scan, connect
or disconnect is done by calling respective completion handlers.
We can avoid use of this workqueue by calling those handlers
from other places.
1) Call connect, disconnect completion handlers in their callback
   functions.
   ex. Call cfg80211_connect_result() in mwifiex_cfg80211_connect()
2) Call scan completion handler after parsing response of last scan
   command in a queue.

After removing the workqueue, variables (assoc_request etc.) and
checks used for mutual exclusion become redundant. Those are also
removed in this patch.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>