mwifiex: fix memory leak issue when driver unload

commit f16fdc9d2dc1e5b270e9a08377587e831e0d36ac upstream.

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.

Reported-by: Daniel Drake <dsd@laptop.org>
Tested-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
[bwh: Backported to 3.2: s/wdev->netdev/dev/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>