ETHER: Make sure SET NOASYNC is effective for Ethernet devices

- Make sure that asynchronous mode can't be changed if devices using
   sim_ether are already attached.
- Add missing DEV_ETHER type flag for the only sim_ether using device
   that didn't already have it.

VAX/vax_xs.c

@@ -130,7 +130,7 @@ DEVICE xs_dev = {
     1, DEV_RDX, 20, 1, DEV_RDX, 8,
     NULL, NULL, &xs_reset,
     NULL, &xs_attach, &xs_detach,
-    &xs_dib, DEV_DEBUG | XS_FLAGS, 0,
+    &xs_dib, DEV_DEBUG | XS_FLAGS | DEV_ETHER, 0,
     xs_debug, NULL, NULL, &xs_help, NULL, NULL,
     &xs_description
     };