Return 0 and do nothing when we get a SIOCSIFFLAGS.

Without this, ifpromisc() always fails (after setting the IFF_PROMISC
bit in ifp->if_flags) and bpf never bothers to turn promiscuous mode off.

PR:	20188

sys/net/if_tun.c

@@ -328,6 +328,7 @@ tunifioctl(ifp, cmd, data)
 		ifp->if_mtu = ifr->ifr_mtu;
 		TUNDEBUG("%s%d: mtu set\n", ifp->if_name, ifp->if_unit);
 		break;
+	case SIOCSIFFLAGS:
 	case SIOCADDMULTI:
 	case SIOCDELMULTI:
 		break;