net: Remove IFF_DOWN flag to compatible with Linux/*BSD

turn off interface by checking IFF_UP flag isn't set:
https://github.com/apache/nuttx/issues/1838

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

net/netdev/netdev_ioctl.c

@@ -990,10 +990,7 @@ static int netdev_ifr_ioctl(FAR struct socket *psock, int cmd,
             arp_acd_setup(dev);
 #endif /* CONFIG_NET_ARP_ACD */
           }
-
-        /* Is this a request to take the interface down? */
-
-        else if ((req->ifr_flags & IFF_DOWN) != 0)
+        else
           {
             /* Yes.. take the interface down */