xen/netback: fix error path of xenvif_connect_data()

xenvif_connect_data() calls module_put() in case of error. This is
wrong as there is no related module_get().

Remove the superfluous module_put().

Fixes: 279f438e ("xen-netback: Don't destroy the netdev until the vif is shut down")
Cc: <stable@vger.kernel.org> # 3.12
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -719,7 +719,6 @@ err_unmap:
 	xenvif_unmap_frontend_data_rings(queue);
 	netif_napi_del(&queue->napi);
 err:
-	module_put(THIS_MODULE);
 	return err;
 }