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>