wwan: core: require WWAN netdev setup callback existence

The setup callback will be unconditionally passed to the
alloc_netdev_mqs(), where the NULL pointer dereference will cause the
kernel panic. So refuse to register WWAN netdev ops with warning
generation if the setup callback is not provided.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wwan/wwan_core.c

@@ -917,7 +917,7 @@ int wwan_register_ops(struct device *parent, const struct wwan_ops *ops,
 {
 	struct wwan_device *wwandev;
 
-	if (WARN_ON(!parent || !ops))
+	if (WARN_ON(!parent || !ops || !ops->setup))
 		return -EINVAL;
 
 	wwandev = wwan_create_dev(parent);