Revert r333365

Even though we don't use it, it appears something else requires it to
be != 0 to work. This breaks tftp boot in loader.efi, so revert until
that can be sorted out.

stand/common/dev_net.c

@@ -190,6 +190,7 @@ net_open(struct open_file *f, ...)
 
 	}
 	netdev_opens++;
+	f->f_devdata = &netdev_sock;
 	return (error);
 }
 
@@ -201,6 +202,9 @@ net_close(struct open_file *f)
 	if (debug)
 		printf("net_close: opens=%d\n", netdev_opens);
 #endif
+
+	f->f_devdata = NULL;
+
 	return (0);
 }