net: mhi_net: Register wwan_ops for link creation

Register wwan_ops for link management via wwan rtnetlink. This is
only basic support for now, since we only support creating one
single link (link-0), but is useful to validate new wwan rtnetlink
interface.

For backward compatibity support, we still register a default netdev
at probe time, except if 'create_default_iface' module parameter is
set to false.

This has been tested with iproute2 and mbimcli:
$ ip link add dev wwan0-0 parentdev-name wwan0 type wwan linkid 0
$ mbimcli -p -d /dev/wwan0p2MBIM --connect apn=free
$ ip link set dev wwan0-0 up
$ ip addr add dev wwan0 ${IP}
$ ip route replace default via ${IP}
$ ping 8.8.8.8
...

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>