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/config.c b/config.c
index 1174eaa..6fc840c 100644
--- a/config.c
+++ b/config.c
@@ -99,8 +99,8 @@
 {
 	struct config *config;
 	size_t size, len;
+	ssize_t rc;
 	char *buf;
-	int rc;
 
 	size = 4096;
 	len = 0;