net: mii: Fix the Speed display when the network cable is not connected

Two different models of usb card, the drivers are r8152 and asix. If no
network cable is connected, Speed = 10Mb/s. This problem is repeated in
linux 3.10, 4.19, 5.4, 6.12. This problem also exists on the latest
kernel. Both drivers call mii_ethtool_get_link_ksettings,
but the value of cmd->base.speed in this
function can only be SPEED_1000 or SPEED_100 or SPEED_10.
When the network cable is not connected, set cmd->base.speed
=SPEED_UNKNOWN.

Signed-off-by: Xiangqian Zhang <zhangxiangqian@kylinos.cn>
Link: https://patch.msgid.link/20250117094603.4192594-1-zhangxiangqian@kylinos.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>