net/rpmsg: fix compile-time warning

Fix this compile-time warning:

rpmsg/rpmsg_sockif.c:381:24: warning: format '%d' expects argument of type 'int', but argument 3 has type 'ssize_t' {aka 'long int'} [-Wformat=]
  381 |                   nerr("circbuf_write overflow, %d, %d\n", written, len);
      |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  ~~~~~~~
      |                                                            |
      |                                                            ssize_t {aka long int}

Signed-off-by: Eero Nurkkala <[email protected]>

net/rpmsg/rpmsg_sockif.c

@@ -376,7 +376,7 @@ static int rpmsg_socket_ept_cb(FAR struct rpmsg_endpoint *ept,
               written = circbuf_write(&conn->recvbuf, buf, len);
               if (written != len)
                 {
-                  nerr("circbuf_write overflow, %d, %d\n", written, len);
+                  nerr("circbuf_write overflow, %zu, %zu\n", written, len);
                 }
 
               rpmsg_socket_pollnotify(conn, POLLIN);