vnet: fix log_debug message format

debug+asan build will fail on initialisation when loglevel==debug

Type: fix
Fixes: 1cd0e5dd533f4209dde453eaa43215e52cd42985
Change-Id: I2005ebf9b95ec3b753c4e6d29337be460c77ffed
Signed-off-by: Georgy Borodin <bor1-go@yandex-team.ru>
diff --git a/src/vnet/interface.c b/src/vnet/interface.c
index 44bcc39..706692c 100644
--- a/src/vnet/interface.c
+++ b/src/vnet/interface.c
@@ -777,8 +777,7 @@
   vnet_hw_interface_class_t *hw_if_class =
     vnet_get_hw_interface_class (vnm, hi->hw_class_index);
   clib_error_t *err = 0;
-
-  log_debug ("set_max_frame_size: interface %s, max_frame_size %u -> %u",
+  log_debug ("set_max_frame_size: interface %v, max_frame_size %u -> %u",
 	     hi->name, hi->max_frame_size, fs);
 
   if (hw_if_class->set_max_frame_size == 0)