wlan: Address excessive logging issues during driver unload

Netdev ops has the valid address until net device is unregistered
and currently same is unregistered at the last during the driver unload.
The calls to net dev ioctls during driver unload, may lead to failure
due to load/unload progress check. And the failure error logs results
in watchdog bark.

To mitigate this issue, make ndo_ops structure members as NULL to
avoid invoking the same during driver unload.

Change-Id: I5552b4ef8289a208b5164fd339d3c5549f58ad2f
CRs-Fixed: 783172