obmc-console: Fix bugprone-narrowing-conversions

For example:

```
/mnt/host/andrew/home/andrew/src/openbmc/obmc-console/build/../console-server.c:769:9: error: narrowing conversion from 'ssize_t' (aka 'long') to signed type 'int' is implementation-defined [bugprone-narrowing-conversions,-warnings-as-errors]
                        rc = read(console->tty_fd, buf, sizeof(buf));
                             ^
```

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I67c158b411f1533ca3b5a62803116e95907e8c5b
diff --git a/test/ringbuffer-test-utils.c b/test/ringbuffer-test-utils.c
index 9172648..56e37ee 100644
--- a/test/ringbuffer-test-utils.c
+++ b/test/ringbuffer-test-utils.c
@@ -5,7 +5,7 @@
 	bool force_only;
 	int count;
 	uint8_t *data;
-	int len;
+	size_t len;
 };
 
 void ringbuffer_test_context_init(struct rb_test_ctx *ctx)