rtl8192: switch to netdev->priv_destructor()

When trying to build from the rpi-4.11.y branch, I'm getting the
following error :

drivers/net/wireless/realtek/rtl8192cu/os_dep/linux/ioctl_cfg80211.c:3464:10: error: 'struct net_device' has no member named 'destructor'

It seems to occur since this upstream commit :

https://github.com/torvalds/linux/commit/cf124db566e6b036b8bcbe8decbed740bdfac8c6



[...]

netdev->priv_destructor() performs all actions to free up the private
resources that used to be freed by netdev->destructor(), except for
free_netdev().

netdev->needs_free_netdev is a boolean that indicates whether
free_netdev() should be done at the end of unregister_netdevice().

Signed-off-by: Bilal Amarni <bilal.amarni@gmail.com>