logging: silence ignorable warning

Since the NMDevice's interface isn't always the same as a kernel
interface name, we won't always have an entry in /proc/sys/net for it.

src/NetworkManagerUtils.c

@@ -605,7 +605,7 @@ nm_utils_get_proc_sys_net_value (const char *path,
 	long int tmp;
 
 	if (!g_file_get_contents (path, &contents, NULL, &error)) {
-		nm_log_warn (LOGD_DEVICE, "(%s): error reading %s: (%d) %s",
+		nm_log_dbg (LOGD_DEVICE, "(%s): error reading %s: (%d) %s",
 		            iface, path,
 		            error ? error->code : -1,
 		            error && error->message ? error->message : "(unknown)");