Signed bit field; int have_hotplug_status_watch:1

Fixes error from sparse:
  CHECK   drivers/net/xen-netback/xenbus.c
drivers/net/xen-netback/xenbus.c:29:40: error: dubious one-bit signed bitfield

        int have_hotplug_status_watch:1;
Reported-by: NDr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/xenbus.c

@@ -26,7 +26,7 @@ struct backend_info {
 	struct xenvif *vif;
 	enum xenbus_state frontend_state;
 	struct xenbus_watch hotplug_status_watch;
-	int have_hotplug_status_watch:1;
+	u8 have_hotplug_status_watch:1;
 };
 
 static int connect_rings(struct backend_info *);