cdc-ncm: tag Ericsson WWAN devices (eg F5521gw) with FLAG_WWAN

Tag Ericsson NCM devices as WWAN modems, since they almost certainly all
are.  This way userspace clients know that the device requires further
setup on the AT-capable serial ports before connectivity is available.

Signed-off-by: Dan Williams <dcbw@redhat.com>