wwan: core: multiple netdevs deletion support

Use unregister_netdevice_queue() instead of simple
unregister_netdevice() if the WWAN netdev ops does not provide a dellink
callback. This will help to accelerate deletion of multiple netdevs.

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>