net: wwan: Add unknown port type

Some devices may have ports with unknown type/protocol which need to
be tagged (though not supported by WWAN core). This will be the case
for cdc-wdm based drivers.

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