rndis_host: Flag RNDIS modems as WWAN devices

rndis_host: Flag RNDIS modems as WWAN devices

Set FLAG_WWAN instead of FLAG_ETHERNET for RNDIS interfaces on Mobile
Broadband Modems, as opposed to regular Ethernet adapters.

Otherwise NetworkManager gets confused, misjudges the device type,
and wouldn't know it should connect a modem to get the device to work.
What would be the result depends on ModemManager version -- older
ModemManager would end up disconnecting a device after an unsuccessful
probe attempt (if it connected without needing to unlock a SIM), while
a newer one might spawn a separate PPP connection over a tty interface
instead, resulting in a general confusion and no end of chaos.

The only way to get this work reliably is to fix the device type
and have good enough version ModemManager (or equivalent).

Fixes: 63ba395cd7a5 ("rndis_host: support Novatel Verizon USB730L")
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://patch.msgid.link/20250325095842.1567999-1-lkundrak@v3.sk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>