ksmbd: Reduce error log 'speed is unknown' to debug

This log happens on servers with a network bridge since
the bridge does not have a specified link speed.
This is not a real error so change the error log to debug instead.

Signed-off-by: Per Forlin <perfn@axis.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>