xde: error on add_unicast should reflect underlying error

[rcgoodfellow]

In this code we just set errno to EFAULT, which is confusing for debugging. The errno should be the underlying error code from e.

opte/xde/src/xde.rs

Lines 947 to 950 in f2614ac

	let muh = mch.add_unicast(mac).map_err(|e| OpteError::System {
	errno: EFAULT,
	msg: format!("mac_unicast_add failed for {link_name}: {e}"),
	})?;