Staging: at76_usb: fix !CONFIG_WIRELESS_EXT build

Fix this build error when CONFIG_WIRELESS_EXT is not set
at76_usb.c:5322: error: 'struct net_device' has no member named 'wireless_data'
at76_usb.c:5325: error: 'struct net_device' has no member named 'wireless_handlers'

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>