wwan: core: remove all netdevs on ops unregistering

We use the ops owner module hold to protect against ops memory
disappearing. But this approach does not protect us from a driver that
unregisters ops but forgets to remove netdev(s) that were created using
this ops. In such case, we are left with netdev(s), which can not be
removed since ops is gone. Moreover, batch netdevs removing on
deinitialization is a desireable option for WWAN drivers as it is a
quite common task.

Implement deletion of all created links on WWAN netdev ops unregistering
in the same way that RTNL removes all links on RTNL ops unregistering.
Simply remove all child netdevs of a device whose WWAN netdev ops is
unregistering. This way we protecting the kernel from buggy drivers and
make it easier to write a driver deinitialization code.

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>