wwan: core: Fix missing RTM_NEWLINK event for default link

A wwan link created via the wwan_create_default_link procedure is
never notified to the user (RTM_NEWLINK), causing issues with user
tools relying on such event to track network links (NetworkManager).

This is because the procedure misses a call to rtnl_configure_link(),
which sets the link as initialized and notifies the new link (cf
proper usage in __rtnl_newlink()).

Cc: stable@vger.kernel.org
Fixes: ca374290 ("wwan: core: support default netdev creation")
Suggested-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Acked-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>