mwifiex: fix potential mem leak in .del_virtual_intf

1) Currently we freeing wdev for each interface in driver unload
path. We may leak memory if user have already deleted an interface.
mwifiex_add_virtual_intf() allocates wdev structure. So it should
be freed in mwifiex_del_virtual_intf().
This will make sure that wdev will be freed when user deletes an
interface and also in unload path.
2) "priv->netdev->ieee80211_ptr" should also be cleared in
mwifiex_del_virtual_intf.

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>