icnss: Allow register/unregister driver execution in serial manner

In cases when wlan driver probe is in progress, icnss driver
rejects wlan driver unregister as driver probe is not completed.
This change enables wlan unregister driver to get scheduled
in icnss driver wq if wlan driver has been regsitered. This
ensures that wlan unregister and register driver gets executed
in serial manner. In cases when driver probe is in progress and
wlan driver unregister get called, unregister driver will be
processed when wlan driver probe is finished.

Change-Id: Iae60ee3d6424b14958e37a649b371cccc6cf06f6
Signed-off-by: Sandeep Singh <sandsing@codeaurora.org>