mwifiex: fix memory leak issue when driver unload

mwifiex: fix memory leak issue when driver unload

After unregister_netdevice() call the request is queued and
reg_state is changed to NETREG_UNREGISTERING.
As we check for NETREG_UNREGISTERED state, free_netdev() never
gets executed causing memory leak.

Initialize "dev->destructor" to free_netdev() to free device
data after unregistration.

Cc: <[email protected]>
Reported-by: Daniel Drake <[email protected]>
Tested-by: Daniel Drake <[email protected]>
Signed-off-by: Amitkumar Karwar <[email protected]>
Signed-off-by: Bing Zhao <[email protected]>
Signed-off-by: John W. Linville <[email protected]>