cdc_ether: flag the Huawei ME906/ME909 as WWAN

The Huawei ME906 (12d1:15c1) comes with a standard ECM interface that
requires management via AT commands sent over one of the control TTYs
(e.g. connected with AT^NDISDUP).

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: David S. Miller <davem@davemloft.net>