staging:wlan-ng: make wlan_unsetup void

this is because this function does a deinit job and most of the
deinit functions are expected to return void. Also this function
doesn't fail anywhere..

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>