usbnet: Set duplex status to unknown in the absence of MII

usbnet: Set duplex status to unknown in the absence of MII

Currently, USB CDC devices that do not use MDIO to get link status have
their duplex mode set to half-duplex by default. However, since the CDC
specification does not define a duplex status, this can be misleading.

This patch changes the default to DUPLEX_UNKNOWN in the absence of MII,
which more accurately reflects the state of the link and avoids implying
an incorrect or error state.

Link: https://lore.kernel.org/all/20250723152151.70a8034b@kernel.org/
Signed-off-by: Yi Cong <yicong@kylinos.cn>
Acked-by: Oliver Neukum <oneukum@suse.com>
Link: https://patch.msgid.link/20250724013133.1645142-1-yicongsrfy@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>